Originally Posted by LukasM
Kiwi has it right, throttle bodies and cams are different, and the map that's loaded in the ECU. Everything else is the same, so upgrading to the same HP won't be difficult once somebody figures out how to crack the ECU. A German company has done it but I haven't been able to reach them. I also think that you will be able to get most of the power with an aftermarket fuel controller, probably even more than a stock F800GS because those run pretty lean. Ignition timing won't make as much of a difference.
I've got both F800GS and F650GS throttle bodies here, will have to see what the difference really is when I find some time in the shop.
That reminds me, what happened to the poster who stuck the F800 cams into his 650? The last post sounded like a call for help.
I do agree you can get more power by running a richer mixture and a fuel controller can do that. No free lunch, you will burn more gas.
All 3 versions have the same 12:1 compression ration, so ignition timing, duration of the cams and and overlap between intake and exhaust determine the difference between absolute power and an exploded engine.
I do not understand the quest for power. All versions give more than you need for dirt and just how fast do you need to go?