i think both kayaks make an excellent pair if you are paddling with yoour girlfriend,
Will, you're confusing the original poster (2 years ago), who wanted to paddle with his girfriend, and the latest poster, Tabatskuri (Chris). His question was whether he'd be better to stick with the Big Kahuna which he currently has, or change to a Wisper for more speed, given that he's quite a big guy.
Chris, I've no experience with Feathercraft kayaks, but since you're quite tall, I'd suggest not committing yourself to a Wisper unless you've had a chance to paddle one and see if it's comfortable. Weight's less of an issue; but as you say, the Wisper seems to be aimed mainly at smaller paddlers. Hopefully you'll get some more definite advice here from someone who uses a Wisper. But it's possible that, if you want more speed than a Kahuna, you may need to think about a Khatsalano (or a composite hardshell).
(Edit: Have a look at the "Adventure Racing" thread under "What boat for me?")