Do not get lost in the Mhz game. If you use a 6 clocker such as the P89C66x from Philips, you get 24MHz performance at 12 Mhz.
Dallas has 4 clockers, they. although are somewhat misleading sinc a few instructions do not execute in 1/3 the "normal" cycle count. Philips just came out with 2 clockers (LPC9...) and Cygnal has 1 clockers I believe. Keil has just made a '51 selector available, limit the search on MIPS, not Mhz.

Erik