aws cc2.8xlarge,
primecoin-0.1.1-hp5 two changes:
1) prime.cpp
const unsigned int nPrimes = (uint64)nTotalPrimes * 8 / 100; // down from 10
2) main.cpp
static const unsigned int nPrimorialHashFactor = 9; // up from 7
You seem to be testing two changes at the same time. The first change will definitely affect performance while the second change shouldn't as people have already pointed out. It's likely you're only seeing the effects of the first change you did.