There is not a whole lot of difference between a 50 and 55 series tire in terms of ride or handing. You may not notice any difference at all, depending on the composition and construction of the tires themselves.
The new tire is going to be taller, and have a larger circumference, but not much. Run the car past one of those radar speed limit signs (if you have those in your area) to see how much your speedometer is off – putting bigger tires on my car actually corrected the speedometer for me 🙂 And if tire clearance is tight already you might get some rubbing on the inner wheel wells.
The other issue to watch out for is ABS is calibrated to work with a specific range of tire diameters, but if 275/50/R20 is what the car came with from the factory you should be ok, it really isn’t that big of a difference.
2) going down in profile makes for a less comfortable ride. Again, less sidewall means less “shock absorption” from the sidewall.
3) changing profile and/or rim diameter will change the accuracy of your speedometer, and may eventually get you in trouble if you don’t get your speedometer re-calibrated to the new rolling circumference of the tire. Smaller profile means more revolutions per mile, your speedo will be “optimistic” about how fast you’re going, reporting a higher number; larger profile means the opposite and you will think you’re at speed limit when you’ll be over.
3) in general, changing width causes more issues than changing height, but sometimes you can do both and wind up with the same rolling circumference.
Summary: not a horrible choice, but I’d at least measure my speedo against moving speed as reported by a good GPS and carry that correction factor in my head, if not get the speedo recalibrated.