Gotcha. If you wanted an exact relationship it would be a study involving more gil than I have so you could adjust STR and PIE 1~2 points at a time xD. Even if you went in 5pt increments, still a lot deeper than I want to get. Also, considering the GC sergeant weapon sanctions 45 attack power, so getting over that on a weapon like the hart is also another mound of gil I don't have ^^;. If someone else wants to take this up, I'm really interested to see the results.

Off-topic from determining stat caps:
For Hart vs Ifrit: .02 DPS would equal the same boost as 1 attack power (for weapon skills), so theoretically you would need 162.5 Attack power, plus the 15 attack power difference. So 178 Attack Power on your gear, assuming you are at whatever stat caps the hart has. Which would be 6 max IV materias on various parts of the gear. o.o;