What would happen if I mounted bigger wheels, like the 15" wheels that you find on the W115 ambulance and wagons for example? The rolling distance would be bigger and the car would be slightly faster, no?
I think your question is about tire diameter, not really wheel size. Normally as the wheel size increases for a car, you reduce the tire sidewall. For example, the diameters of all these tire sizes I use on r107 chassis cars have about the same stock rolling diameter:
205/70/14, 205/65/15, 205/55/16 and 225/50/16 (or maybe a slightly taller 205/60/16 and 225/55/16).
The speedometer is probably calibrated for a stock diameter tire size. When your tires are new, you might go slightly faster than your speedometer says, and when your tires are old and bald, you might go slightly slower than the speedometer reads.
The larger tires may give you "longer legs" and a little more highway speed as long as you can get there. The acceleration may be slower though as you are effectively reducing the gear ratio. You could obtain a more dramatic affect by changing the rear differential to a lower ratio. Lower differential ratios will make acceleration a little slower but give you lower highway RPMs, but also probably less passing power.
If your car is still capable of hitting the top speed, it's probably running well and you should probably leave it as is and enjoy it. That's my opinion.