The Silvia Varietta is probably the worst car in the game - no speed, no handling and no tuning (271hp tops).
The CR-X delSol is also a convertible, in the same style as the SLK230 (or the other way around, rather).
Technically the Tommy kaira ZZ-II is a convertible, as you can lift out the mid roof section (apparently).
You also missed off the Tommy kaira ZZ-II, the Vauxhall VX220/Opel Speedster and the Lotus Elise 190.
My favourite is probably the VX220/Speedster.