If there's no difference in the suspension for the convertible vs. hardtop, i would recommend the F-sports springs and sways. They made my IS350 handle much better. I bought mine from Westminster Lexus.
2010 IS350 Obsidian Black/Black interior
2003 IS300 5-spd SportDesign Black Onyx w/LSD | 4.1:1 Final Drive | Borla Exhaust | Megan Racepipe | HKS Cam Gear | JIC FLTA1 Coilovers | L-tuned Sways | Fidanza flywheel | PowerSlot Rotors | Goodridge SS Brakelines | PIAA 6600K | CDD delete | Teck kit | Gone: EMU, SRT intake, B&M shifter, TRD Grill