Micheal,
when I was about to build my 12's, I spoke to a guy who has dual OB 8's ( 2 per H-frame). He really thought if I were only starting out with 1 H-frame in a dual driver config, it would be wise to go with 12's.
I don't know what speakers you are going to to use these with, but as mentioned, sensitivity is going to play a role here.
It wasn't clear from your post, but if you are only planning on doing a single H-frame for now, a dual 8 probably isn't going to provide a whole lot of output,. If you are planning on 2 dual H-frames, the fellow I spoke to was very happy with his (2 dual 8" H-frames). I could probably dig through my PMN[s and find out who it was for you if you like.
I'll add a bit more , remember, that even though you may still be able to reach your desired SPL levels, you will be signifigantly increasing the workload of each driver by using less drivers, especailly if you are only thining of doing 1 H-frame to start with. Going with all the extra drivers is not so much about the extra SPL, more about decreasing the work each driver is doing to reach a given SPL and thereby increasing the quality of the output.
jay