As far as compiling, as my original post states, the clang-compiled kernel self hosts. Before I had SMP working, it took about 17ish minutes to build (the result of that build was a kernel with SMP). I'm using a self-hosted Linux kernel that was built with Clang right now. I don't have hard numbers, but compiling doesn't seem to be too much slower. This isn't an optimized kernel build, though.
I've been unable to find a usable kernel test suite. I've tried a few, and been disappointed by all of them, most of all by the Linux Testing Project.
The best test suite I've found so far is a debian package called posixtestsuite. It's a few years old, and looks to have been written by a bunch of Intel hackers. It has a number of desirable features, such as the ability to be compiled without hair pulling, and documentation.
I'm open to suggestions regarding broader performance testing solutions.