If you're going to go by RAW to say an oversized light weapon is a one-handed weapon and thus can be used for power attack, then you should go by RAW and say the Superior TWF removes the -2 penalty, but since that doesn't exist for you, it does nothing to help with the -4 penalty. Then once you add the -2 penalty for oversized, you wind up with a -6 penalty total.
If you want to go by RAI for both, then I'd say it would take 2 off the penalty, but I'd also say that oversized light weapons still don't qualify for PA, since they're not made that way. Why should a smaller creature wielding the exact same weapon get PA when the correct size creature does not? (But then again, I'd probably allow PA for light weapons anyway since I play with a group that doesn't optimize in the least)
Of course, few DMs go strictly by RAW or RAI. I'd ask the DM and see what he/she wants to do.
Good luck!