Exactly, well observed, .21s are the most evolved 1/8th engines, just as the .12s are the most evolved 1/10th engines.
Having said that, the OS 18tz is a pretty special engine. It is almost as good as a racing .12 and because its an .18 it has a lot of torque. You can gear higher to get the same top speed as a .12 with no real compromise. Here are some tips from someone who has fitted the OS 18tz to the RS4...
http://www.rcuniverse.com/forum/m_45.../printable.htm