Um...doesn't it defeat the purpose of putting the tank in the wall if you're going to mount the bowl on a 12" rough-in anyway? It's not like you are going to design a toilet that has a trap mounted forward of the bowl...
Personal advice: if you want a toilet that flushes well, has great reliability, and has easy-to-find parts now and into the future, stay away from the Kohler. And even if the bathroom is small, unless you have a real structural barrier (like a door that swings into the toilet), stick with the elongated. I have a great round-bowl Toto Drake in a tricky spot, but in retrospect I might have just as well installed the elongated; it wouldn't have made that much of a difference appearance-wise.
Also, the overall "feel" of the size of the toilet is dependent upon more than how much it extends into the room. Our Carlyle II is actually deeper into the room than the toilet it replaced, but because of its overall size, it feels smaller and much less intrusive. So don't just focus on the one dimension. An Ultramax II or Carlyle II doesn't feel like a big toilet when it is installed, for example. The round-bowl Drake is one of the smaller toilets out there in terms of intrusion into the room, but the Carlyle II doesn't feel bigger.
I'm using Toto as an example because your default choice should be Toto. Start with them and see if they don't have what you want, because you can find them at comparable prices to the other junk out there and you're almost guaranteed to be happy with them now and over the long haul. Look around this forum (and in the user reviews section of Terry's green toilet reviews link above), to see why.