Inner diameter of stock downpipes?

[Freza]

I don't believe for one second that you'll gain any BHP, the main difference i would imagine would be a little less weight, but even then it won't be enough to be noticeable for normal road riding.

A long time ago I tried my friend's FZS600 with some aftermarket downpipes and power increase in low to mid rpm was pretty noticeable.
He had a Yoshimura slip-on.
Don't know which downpipes those were, but they were wider than mine (stock).