XR 250 exhaust leak

Hey all, I have a xr 250 that has a duel exhaust coming out of the cylinder head. ( Sorry if Iim using the wrong terms, im not much of a tech). Well anyway, one of the points were the exhaust connects to the cylinder head has a HUGE leak, the gasket fell out and ive been trying to use exhaust putty for a while and its not working... Well my question was when the gaskets are installed again, How much more power will i get from my bike, I never rode the bike with a tight seal on that exhaust. Will I notice a small or significant change is speed, power, anything else? Sorry im not much of a tech and know a small deal about engines, I would assume it would gain some power but thats why im askin u guys. Thanks, :)

What damage would be caused if i ran it that way for a few months of weekend riding?

Alright thanks for the advice.