Hyperthreading doesn't turn a 4 core chip into an 8 core chip - even if software reports that it's 8 core.
Don't expect a hyperthreaded 4 core i7 system to be twice as fast as a non-hyperthreaded i5 at the same clock rate.
At best, assuming a fully multithreaded application, the HT i7 have be 20% to 30% faster than the i5.
At worst, the i7 could be somewhat slower than the i5 due to scheduler issues. (If the scheduler puts 2 threads on virtual cores on the same physical core - they'll run slower.)
