SEAT: The stock seat is like a plank after a couple hundred miles, usually after 200 i cannot feel my arse and feck knows how i did 600 miles in one hit but i can tell ya my arse felt wrecked, i have the ergo and they offer more comfort but there slightly thicker so means i cannot touch the floor so never use and still boxed, I've adapted and apply some pressure with my feet to ease my arse off the seats on long hauls.
SAT-NAV BRACKET: i use the ktm bracket for my tom-tom rider and its fine, never had a problem, its positioned above the clocks and does not vibrate.
EXHAUST: you can de-bung the original to sound like my akro, the original has the baffle in the bottom of the can and bods on the forums use a dremel to cut the collar and pull it out from the bottom, alternative look on ebay, any end can that fits a SDR will fit the GT, i won a brand new akro that was listed for a SDR at a bargain price.
BEST MOD: 40T rear sprox, works really well with the shifter because stock gearing makes the Qshifter change feel delayed and the 40t sprox seems to speed up the operation of the Qshifter, rich@redine also agreed with me on the shifter change being delayed.
HAND GUARDS: if you buy the Ktm adv ones they might touch on lock, i have TT rises and my bars are in the position nearest to the tank (i think theres 3 positions on the top yoke) and there absolutely fine.
Theres a couple of dedicated forums with a GT section: Adventure rider / ktmforum.co.uk (this one is particular quite) / ktm super twins (plenty of info)
Get rid of the emissions crap under the rear seat and it frees up loads of space, I've done quite a few mods and they have made the bike a real joy to own .