The natural comparison point to the 135is Convertible I just looked at is, of course, the same year 335is. Indeed, since the two share a fair amount of architecture and a similar recipe, on paper it’s a bit strange that they were offered at the same time. But though the 335is and 135is looked similar, there were actually quite a few differences between the two packages.
For starters, though they were both turbocharged and rated at 320 horsepower, the 335is package retained the N54 motor to do so. I’ve never quite understood BMW’s logic on this one, but clearly there was something about the N54’s power delivery that they felt was superior to the N55 single-turbo. The reserved the higher-output N54s in late production for the 1M (335 horsepower), the Z4 sDrive3.5is (335 horsepower) and the 335is (320 horsepower). The N54B30TO also got a 7 second overboost of 40 lb.ft of torque on top of the 332 that was rated in this car as stock. As I described in the 135i post, the easiest identifier for these cars was the wheels, and on the 335is you got massive yet delicate Style 313 M Double Spoke wheels, 8″ in front and 9″ in rear, with more aggressive offsets than the E8x got. Hunkered down on M-Sport suspension, the 335is also got a unique M aerodynamic body kit, and the transmission was borrowed from the M3, along with additional cooling for the engine. They carried a less restrictive exhaust system. Coupes went so hardcore that, like the 135i, they dropped the foglight option. The convertible variant was apparently judged to be a little less track-ready, as so like today’s Le Mans Blue Metallic example, you could retain the foglights: