Pay attention to the fit of the ball joint into the trailing arm. When I pulled mine, the joint shaft had obviously been orbiting in the oversize hole. The non-tapered shaft was quite sloppy on my '07 GS (21" Woody, Wunderlich triple clamp), enough so that BMW offered to replace the arm, though the bike was out of warranty. When the new part arrived, it had the same sloppy fit. I solved the problem with a section of brass feeler gauge as a wrap-around shim.
The stiffer triple clamp will give you a lot more than just clearance for your 21" front. At least with the Italian Wunderlich clamp, the bike steers much sharper and is less forgiving than with the original softer lower clamp. Feels more rigid, like a race bike, and requires more of the rider, like a race bike. Fun, but like everything else, you get something and you give something up.