I took the pipe off my nightster to try and fit the rear footpeg bracket. When I put it back together something just doesn't seem right when I'm riding it. Will a bad exhaust gasket make a huge difference on the way the bike performs?

 

i think it can a leak could cause a more open exhaust flow like upgrading the pipe from stock except you have to tune everything after i would change gasket and re-torque fasteners

 

A leaky exhaust gasket will typically be noisy and cause a lot of popping/backfiring.
It will certainly NOT improve performance.
The manifold gaskets should be replaced every time the exhaust pipe is reinstalled.

 

So I replaced the exhaust and as far as I can tell on my test ride it made a huge difference. Surgeing smoothed out. I'm happy again.

 

^^^^exhaust gasket*