I did that for a while - see here and here.
At the time, I was working on GTK+ and X11 hacks for a Linux cell phone distro, and every time I touched something on such a low level, it triggered rebuilding all kinds of things. One of my colleagues never did complete builds because, on the computer the company supplied with the standard compile options, it took five hours.
I had all kinds of crazy hardware sitting around at home, so I ran benchmarks on some machines while I coded on others, and you can see the results at the links.
For what we were doing on Ubuntu, once I maxed out CPU utilization - which you can do really easily with the -j argument to make - the bottleneck seemed to be the disk.
But then the company had big layoffs, so I was out the door, and didn't finish scoping that all out. I had a lot of data and interpretation I didn't post on that blog, too.