The affects of cache is limited, over a certain amount it doesn't add much if any speed increases because of other factors. If the bigger cache chips are the same form factor as the regular ones and you are handy with surface mount chips then swap some out and benchmark it. If the benchmark program fits completely in the cache then you will see exactly how fast the CPU is.

If you wanted to see major speed inprovments in computers then make the RAM speed faster then the FSB of the CPU so you don't even need cache at all. Somebody years ago shipped a PC (386 maybe) with SRAM for main ram because for a brief instant it was cheaper then the normal DRAM used. SRAM was what was used for cache back then and much faster then DRAM, so the CPU ran with no wait states and was a screamer.