To what extent can frequency improve speed?
No, it isn't. I've heard about the NXP ARM7TDMI at 60 MHz outperforming an Atmel at 72 MHz. IIRC the reason was that the Atmel had to add wait cycles for reading Flash memory. The NXP had to add wait cycles as well, but read 4 words at a time, so the wait cycles were only needed in 1 read out of 4.  

Wait cycles are typical for distorting the linearity of MIPS/MHz. For instance at 30MHz you may not need wait cycles, but you may need them at 33MHz, which will probably make the 33MHz version slower.