Not threadcapping, but I'd simply go with TinyXP v6, although I prefer to use eXPerience's service scripts.
What you are using, is exactly what you described. A slimmed down version of XP. Any XP install can run fast and have a small footprint, it depends which services you disable, which components you remove, and a few other things (unnecessary files removed).
I uploaded eXP's scripts awhile ago, they should still be posted somewhere. PM me if you have any questions regarding this or anything about slim XP installs.