Yes, 195/65-15 tires and 275hp are not a good mix. The super tall rear ratio helps a bit, but an E420 can be a real handful in wet weather if you're the least bit ham-fisted with the throttle.
Most E420's I see have 16" wheels with 205/55-16 or 225/50-16's on the them. The eight-hole wheels look great and can be found at pretty low prices for knock-offs or even take-offs for OE wheels. Then the 15" wheels are super for snow tires.
1998 C230 "Black Betty"