RB, I've seen 20" 132's in person as well and they do look amazing!
TX5, I did browse through those wheel threads. Lots of info in them for sure on OE and aftermarket wheels. This (bigger style 63's) was just something I had never seen anywhere prior.
Unsprung weight is any weight not carried by the suspension of the vehicle. Basically everything from the springs/shocks to the tires.
TiAg, yes they were available OE only in the sizes you mentioned, they are what I have on currently.
On unsprung weight, yeah, they do look heavy as hell. But the rest of the truck is not light anyway so screw it. :P
Now that it's been mentioned it seems to me (personal opinion) that the E53 looks best with weighty looking wheels. With its huge flanks and black plastic around the wheel openings making the openings look bigger, it takes a chunky wheel to look good in there.