Sorry for the delay responding, I was actually out riding in our beautiful 70 degree weather.
There are two bar risers available for the GT. The HeliBar unit moves the bars back 1", at the stock "tall" setting. The Suburban Machinery units move the bars back 1", but also have five height positions. The lowest is equal to the stock "tall" position. The second and third positions add 9 mm each (0.35" or 0.7"). The fourth and fifth positions are also 9 mm steps, but are too tall to allow the throttle cables to reach comfortably.
I've been running mine in the midway position and would prefer them a little higher. But I just picked up my bike from getting the air box retrofit, at which time they discovered my throttle cables were binding. They replaced them, although the old cables work fine while on the bench (of course). That's the only time I've heard of this happening with the risers, but I figured it was better to mention it.
Now that I have the old cables off the bike, I'm going to find a local cable manufacturer and see if they can make me up a set that's 2-3" longer so that I can run the SM risers at full height. I imagine it will be easier to use all the BMW fittings and just replace the cable and housing with longer pieces. Obviously I don't have any idea as to cost yet.
As for peg extenders, your choices are Suburban Machinery or Verholen. I put the Verholen 40mm (1.5") peg extenders on the day I got the GT. The SM extenders are fixed, but the Verholen extenders are two-piece with a serrated interface which allows a bit of fore-and-aft adjustment if needed. But I found that once they're set you don't really need to move them.
With those and the seat in the high position I actually had slightly more leg room than the LT. But it's much easier to drag the lowered pegs in the corners, so if you're an aggressive rider then you might want to consider that. When I got more comfortable on the bike such that I could drag the pegs at will, I ended up pulling them off. I haven't really noticed any comfort issues without them, but my seat is always in the high position, plus I did have it built up slightly by Rocky Mayer.