I currently have the stock tires on my 2003 Acura TL Type-S(P215/50R17). A friend of mine has 4 P215/55R17 and I was wondering what a difference they would make or if I could even use them?

 

the sidewall or profile height is a little taller than stock. it has a variance of over 3% which is too much and your spedometer will be off.

acurazine has a tire calculator that you can use to compare tire specs.
my favorite tire calculator is from 1010 tires tho which can be found here
http://www.1010tires.com/Tools/Tire-...0R17/215-55R17

 

To OP you can use any tire combo you want, its the difference in gearing what will affect the outcome being a faster tire or a slower tire.
Smaller circumference tires will give you slightly better acceleration while Larger tires will give you higher theoretical Top Speed.

So staying between the stock circumference might be a good idea.