ok i made a few test, im running an i5-2500K @ 3.30GHz with an linux 64 OS

this are the average (about two minutes) hashrates i got:

|12 miller rabin tests  | 1 miller rabin test | fermat test           |
| (default miner)         |                              | (fork from above) |
------------------------------------------------------------------------------
|        84492                |        89620            |      78685              | --max-prime=10000
|                                  |                              |                              | --sieve=4194304
|        63276                |        64267            |      55844              | without exta
|                                  |                              |                              | options

so the fremat test doesn't got an improove on my system,
but thats maybe releated to the fast gmp implementations of the miller rabin test
(i thought fermat test should be theoretically a bit faster than miller rabin)

but decreasing the max-primes an seting MR_TESTS 1 got an reasonable increase