I have a PAL Wiikey mod, and I've found that Gamecube games of all 3 regions work really well. Most of the games I tried are US format as well, and all games were working fine.
Yes, you can just burn the iso/gcm images with IMGburn, but its best to create multiple Gamecube games to fit on a single DVDR. The piece of software you require is in the Wiikey Base 1.2 setup disc. Also, shrink the 1.5gb images down with the GCMUtility program (read the readme), and you can fit as many Gamecube games as you can onto 1 DVDR until its full.
I've read that the Wiikey has a problem with some audiostreamed games, but I have not yet encountered them.