From what I've read, Kaelten, the Curse Client is, for Windows at least, being developed using the Windows Presentation Foundation. My question is thus: assuming this is so, would that not mean its being developed for the .Net Framework? If it is, then you could probably port most of your code base to other, non-Windows operating systems using Mono, and perhaps finding something that'd let you tweak the UI and some more specialized bits of the code for your target non-Windows platforms. Yeah, the first "public alpha/beta" versions would be a little "ugly" and maybe a tad slower than their Windows cousins, but they'd placate the community while giving you a platform off which to jump into improving the non-Windows user experience.
I can back Qt programs being a little slower than their non-Qt cousins, as well. I think this may have to do with added overhead from the framework itself...or poor code on the author's part. Unfortunately, it seems to be the best (if not necessarily only) choice for developing non-managed GUI applications using C++ with any form of efficiency. This is all my own personal experience, of course, so your mileage may vary.