There are quite a few testing tools and verifiers at Tony Graham's XSLT Testing Tools page. If you haven't looked there, it's a fairly comprehensive list.
Using Saxon in schema-aware mode will catch many common errors.
You've already discovered Mukul Gandhi's XSL Quality tools, which support user-added extensions. On the xsl-list run by Mulberry Technologies a while back, several other people contributed ideas for new rules also. You might also get help asking there.
Stylus Studio, Oxygen and xmlspy have profilers for run-time performance.