No performance difference that I've noticed yet. I have SBSettings and LockInfo installed and that's about it (MyWi as well but that's just a regular app that wouldn't affect performance unless it's running).
I stay away from themes and changing icons and such, I think those type of things could affect the performance of the phone.
No it isn't. Where's your proof of this? It's the applications and tweaks, not "jail breaking". Jail breaking just 'unlocks' your phone to customization, it's not a program that constantly runs to hog resources and slow down the phone.