Inner diameter of stock downpipes?

[darrsi]

Noticeable by sensing stronger pulling, not by noise.

Not disputing it, but even a K&N filter makes the bike feel like it's been massively improved, and i will vouch for that too, the throttle response and pulling as you say certainly feels better, but in reality there's not really much BHP improvement, it's just a feel good factor.
Bit like these 125's you hear that sound like they barely have an exhaust on, they sound like they're going fast, but they're not really.