If you fit the 50 profiles, they will be 3% smaller on diameter - should still be slightly smaller than the worn 55s. You should find your accceleration will IMPROVE a little, but your actual top speed will be lower as the car will be slightly lower geared overall.
Your speedo will read 3% higher too - as suggested it would be best to check against a GPS as speedos aren't the most accurate of devices anyway.
Alloy wheel sellers will usually recommend staying within about 2.5% of your original size, so you will exceed their limit, but I wouldn't see it as much of a problem. You will, of course, have slightly less ground clearance, but that shouldn't be a problem unless your suspension is lowered and you live near loads of speed bumps!