[QUOTE=Sacrificial]My 2 cents?
Let's be sure to get our nomenclature straight, by lowering I mean just that: lowering the ride height. It doesn't necessarily mean coilovers, springs, or whatnot.
For the IS300, lowering may reduce your ride quality quite a bit. This is because just about all the Japanese-built springs/coilovers are actually calibrated for the lighter Altezza RS200/AS200. These RS200-calibrated rates are usually insufficent at keeping the front end of the IS300 from "soft crashing" on its bumpstops. (Some of you may volunteer that both ends feel worse, but the fact is that butt-dynoing which end of the suspension is causing the rough ride isn't as straighforward as one might imagine.) To reduce the suspension travel of the IS300 yet keep it off its bumpstops over common pavement irregularities, a much higher spring rate and/or higher rising rate is necessary. The only time the ride of a soft-sprung lowered car might feel acceptable is at-speed on a reasonably smooth road, where vehicle inertia is doing most of the body control. To deal with the "soft crashing" at low-to-medium speeds, one can turn up the damping adjustment (which is combined compression & rebound on most shocks and coilovers) to the point where the compression damping is overused in its body control capacity. While it is effective at reducing "soft-crashing" and endows the suspension with a feel of composure, it actually decreases grip and will send harsh vibrations right through the cabin over washboard-type surfaces. To be completely fair, turning up the compression damping past the optimal combination of body control and grip makes the breakaway characteristics much milder, hence why it's often regarded as a "drift tune". Not a lot of grip and hence not a lot of real speed, but extremely easy to manage breakaway and the body just about never misbehaves.
Even apart from the damping overuse issue, lowering will usually also reduce your actual handling performance through excessive camber. This is because 1) the suspension features a fairly aggressive camber curve, and 2) there isn't much range to the camber adjusters found on the car. Lowering the car to any significant degree (1" perhaps?) will dial in so much negative camber that the car won't be able to put down a full contact patch in a straight line nor in the midst of even hard cornering. Any of you who might disagree with this can prove this to yourself if you spend ~$120 to obtain a tire pyrometer and learn how to use it.
The excessive camber issue is worsened if the car is equipped with bigger sway bars. Body roll normally hurts performance on the stock car by putting the wheels into positive camber, but in this case the more positive camber to counter the lowered suspension's excessive negative camber the better. But the sways take that away too. Of course, this is all in addition to TLLTD effects.
Lowering will also do a real job on your tire wear. The excessive negative camber will eat up the inside shoulders of your tires, worsening with higher-performance (read: firmer sidewall and softer tread compound) tires. While this effect may not be as profound with long-lasting, lower-performance tires, check out one of the Falken Azenis Sport that was mounted on my rear axle:
(Don't worry too much on my behalf; this tire has 5 plys in the tread and 3 plies in the sidewall. As far as I could tell, I only ate completely through two plies before I chucked it.
I think it's kinda funny how the tire is no longer a cylinder, but more of a truncated cone. For those of you who met me at the last car meet, note how mildly my car was lowered, if you even noticed that it was lowered at all. Since my installation of coilovers, my rear camber adjusters have always been maxed out to counter as much negative camber as possible. My static rear camber then was at around -1.1 degree. This undeniable evidence of a truly screwed-up contact patch prompted for the purchase of a tire pyrometer, which told me with surety of how much contact patch I was squandering. Being that I was already maxed-out at the end of the camber adjuster range, I started raising the car using the pyrometer to guage my progress in reclaiming my contact patches. By the time the pyrometer told me that my contact patches spanned across the entire tread width, the ride height was here:
Not very lowered, is it? But it now grips like a mofo to the point the car's dynamic capabilities are currently beyond my ability to fully exploit it. But I'm working on it. 8)
Many people believe that lowering the IS300 enhances handling. This isn't false from a seat-of-the-pants perspective, since lowering the car changes the perception of speed (the lower you sit to the ground, the faster the apparent speed) and reduces the time it takes for the car to "take a set" during corner entry. This owes to two phenomenons: 1) the fact that there's a whole lot less suspension travel to dive through before the shock stops diving because it's sitting on the bumpstop, or 2) due to miscambering, there isn't much contact patch to really laterally accelerate the vehicle much, so the "set" arrives at an artifically low point. All this gives the impression of a quick "set" when you steer into a corner, like that of stiff sway bars. It feels sporty and nimble, but you'll probably get lower skidpad numbers than even a stock car.
Another thing that leads people to believe that lowering enhance's the IS300's handling is the fact that the car seems understeer less. Remember my tire above? The reason the stock IS300 understeers as much as it does is because in maximal-performance cornering, the front wheels go into positive camber way before the rears if the car is aligned to factory specs. Good engineering places the "zero" (point equidistant from the ends of the range) of the camber adjusters at around the factory spec mark. Another way to understand what I'm saying is that the rear tires are simply negatively cambered more. Via lowering, adding a heckload of negative camber past the point of good performance on both ends produces front wheels that are excessively negative-cambered, but the rear wheels are even worse. (Remember that it was my rear tire that I showed you all for dramatic effect. My fronts weren't as bad.) The car seems more neutral because you're taking more contact patch away from the rear wheels.
I guess this is more than 2 cents. Take it for whatever it's worth.