I actually think the SpD IV influences HP power most.....but I think that SpA IV is more important than Hidden Power power, because since you're typically using it for coverage, it makes no sense to have the other moves have less power. Take your Clamperl...it's preferable to have Surf hit harder if you can't have both of those things. So you should aim for high overall stats than HP power.
From the same thread I linked you to, I think this might help you:
Since it seems to answer many of your questions, i would personally recommend bookmarking it for future reference.Hidden Power IV Combinations:
*IV's are listed HP / Attack / Defense / Special Attack / Special Defense / Speed
*E = Input any of the following even numbers: (2, 6, 10, 14, 18, 22, 26, 30)
*O = Input any of the following odd numbers: (3, 7, 11, 15, 19, 23, 27, 31)
*Trick Room combinations: Just replace the Speed IV with a 2 or 3 depending if its even or odd to remain a power of 70.