Derek, my wife and I together are in the high 400s, she won't tell me her weight so I don't really know but I figure we are pushing 500 lbs. I don't have a K12GT so can't comment on that but I have ridden Gold Wings a lot so that's where my comments are from:
1) Don't know if you've ridden with her on the back before BUT, you'll really notice it. I've ridden with 200+ people and 150 people. At 150 I start to notice folks and above there it gets much more noticible.
2) The Gold Wings GVW (and max weight for riders and gear) is less than the K12GT or LT. Yet I constant see a combination of large folks and LOTS of luggage. Easily exceeding the recommend limit by 100+ pounds or more. This often includes some rather large tongue weights on trailers.
3) You need to set the suspension up to maximum.
4) You may find that steering has been compromised a lot because the total Center of Gravity has shifted back more.
5) You really need to try out the bike to find out how you'll end up sitting. With my wife, my back touches her front which sounds great but in warm weather it isn't and it doesn't give me any ability to move around.
6) The bike will "wallow" somewhat even with this. Keep your tire pressures up but do NOT be aggressive in any fashion.
7) I'd suggest you test sit on a GT, an LT, a Gold Wing, perhaps the new Connie and heaven forbid, a full dressed Harley to see what the comfort is for both of you. Note the GL1800 Gold Wing can be ridden pretty agressively (Likely like an LT and both will out ride a Harley). But if the missus ain't happy you get slapped upside the head so check it out.
Best of luck.