Your 2 12W7's will be underpowered but adding a second 1000/1 will definately make you happy, if you only plan on 1 amp do yourself a favor and go with 2 10W7's. I was the USACi region 10 points champion 2 years ago. I was running 1 13W7 off of 1 Kicker XS 100 (birth certificate 1304 watts at 1 ohm) and in a downfire ported box was doing a 149.3 on the floorboard open vehicle, 145.2 on the dash sealed and a 159.8 port wars and the sound quality was excellent. You will be very happy with whatever size W7 you decide to go with.
I agree any size w7 will sound nice. But is there really such a thing as underpowering. It doesn't matter how many watts your amp puts out, clipping is clipping and it will destroy any speaker. Keep in mind the only time an amp puts out its full rms power is at full volume or right below clipping. So if underpowering a speaker was really that big of a deal (minus the clipping) then why doesn't every speker blow after a little use? The point is, he can push 2 12W7s with a 1000/1 just fine and still get louder than the 2 10s.
Anyone know the output potential of 1 10W7 in a custom H.O. wedge box runnin off a 500/1 vs. 2 10W7 in a differant box like sealed or ported running of a 1000/1 cause the H.O. is one of a kind box and its huge for one sub. Or maybe vs. 1 12w7 in a sealed box? I know it will be very efficiant in this box but not sure how they compare to the others i mentioned.