Originally Posted by
marqueemoon
Totally depends on the setup of your current bike and how you like a bike to fit and handle.
If you're running a slammed -17 100mm stem on your current bike with a fistful of zero setback post, yes you could probably size down and make it work. If you're running a 130 stem with 40mm of spacers and your saddle slammed back on a sky high mega-setback post the next size down is going to be too small.
This is a good response. Some of the posts got off track - the OP is talking about getting a size smaller than he has now. As far as performance suffering, it won't if the bike is setup right. Pro's generally ride a very small frame with a 130-140 stem and lots of seat post showing. Many also use a -17 degree stem for bikes with tall head tubes.
Between stem lengths and angles, seat posts with various setbacks, and seat post heights, there is lots of room to adjust.