I'm running a game, and house ruled that the Double Slice feat give the full power attack bonus along with the full strength bonus for the offhand. I thought I was being clever, but a quick search on the boards let me know that I wasn't the first with that idea. Despite the minor blow to my ego, this actually works in my favor as I can ask all you homebrew guys and gals if you've done this, and how it worked out?
For clarificaiton my custom feat reads:
Your off-hand weapon while dual-wielding strikes with greater power.
Prerequisite: Dex 15, Two-Weapon Fighting.
Benefit: Add your Strength bonus to damage rolls made with your off-hand weapon. Additionally, if you have the power attack feat, you may remove the -50% damage penalty for attacking with an off-hand weapon.
Normal: You normally add only half of your Strength modifier to damage rolls made with a weapon wielded in your off-hand. You only add half of your power attack damage to power attacks made with your off-hand.