AH, you have the T9512-04. Okay that makes more sense now. That's a single voice coil, 4-ohm sub. If you only have one of them (which I believe to be the case), then it cannot be wired for 2 ohms. Only 4. I believe those extra terminals are there so you can wire it to another sub. So wiring it the way you did, you're right, you won't hear any difference, since nothing changed by adding the extra wire.
You will only get 500w power to that sub with that amp. That amp can only deliver 500w into 4 ohms. If you want more power to that sub, you need a bigger amp. Or maybe find someone willing to trade for a dual voice coil version of the sub.
But still, you need to properly set the gain.