I'll answer you the best I can.
It's not a big difference in terms of HP.
Termis and a ECU tune claim a 10% increase in HP, I was really pleased with this until I saw an article on the web that did an actual dyno test. It added like 5 or 6HP if I remember correctly, which is underwhelming, really. I'll share this here as soon as I find again.
More important that the HP numbers though, is that the tune makes your air/fuel ratio richer. This means (sorry if you already know all of this) that there's more gas in the mix that's injected into your cylinders. The opposite of a rich condition is lean, which means there's more air. Our bikes, because of emissions regulations, come from the factory in a lean condition. Once you get the ECU mapped, you'll notice your mileage go down because you are burning more fuel, you're running richer. This may also mean that you will not pass emissions testing if your country or state tests motorcycles. Mine doesn't.
When you are running an upgraded exhaust without a tune, the engine gets leaner. Traditionally this is bad for the engines as a lean engine burns hotter, is less efficient, and has increased wear and tear. This was especially true for air-cooled engines. You had to re-jet your carburetors if you changed your exhaust. But our engines are EFI, water-cooled and modern metallurgy provides us with super hard cylinder walls and pistons! So...
The only way to prove this is a reliability test. You pretty much strap on an aftermarket exhaust, leave the stock ECU and test to failure. Or non-failure, which would be reliable. The engine may be able to handle minuscule lean conditions.
Our motorcycles are so new that nobody has experienced failure with modifications. That's what these forums are for, I think. I'm thanking in advance the guy who reports that so-and-so exhaust ruined his engine.
Even though I secretly lusted after Akrapovic, this is why I still went with factory-endorsed Termis.
