Rear derailleur cage bigger than cassette necessity I use a Shimano Ultegra 6600 rear derailleur with short cage in a 11/28 cassette. The derailleur is at the end of its useful life and would like to exchange for a cheaper one, since I intend to exchange the complete groupset in the future.

I found a really cheap 105 5701 rear derailleur, but only the medium cage version is available, that is compatible with cassettes up to 30 teeth. If I use that medium cage derailleur with my 11/28 cassette, is it possible I'll have any performance issues?
An RD-5701 rear derailleur should work just fine as a replacement for an RD-6600.  I've used an RD-5701 with an 11/28 cassette for the last 6-7 months with no issues, including swapping my crankset from a 50/34 compact to a 53/39 a few weeks ago.