How much horsepower does a stock FLSTFB have?

Not much for a 1600cc 65hp bike but if you spend 10grand you can have double :-) Not very cost effective for double the horsepower.

Best to leave it stock as they go ok for what they are as they are only a cruiser, plus if you upgrade your engine you better upgrade those brakes as you may have issues stopping that big heavy beast. If you need speed handling and good brakes better buy a sportsbike :-0