I think you're thinking overkill.
And, as someone who has the Mameroom cabinet, you still don't need to do "what they did at Retroblast". Just make some measurements and get the right screws.
My point is, I've got 5x7 Sony Xplods and a Creative 2.1 system in my cab and it's more than just fine by me. From the sounds of it, you want something more than that, but frankly, I think you should take a step back and do some asking around--of yourselves and others.
For 1, not all games are going to be fully utilizing either stereo or much of the subwoofer. Just remember that.
2 - mounting speakers above the speaker covers in the mameroom ua2 cab is cake. Like I said, measure, get your holes lined up and get screws that won't go all the way through. The Sony Xplods have a fairly translucent red speaker to them and you can sort of see that through the grill in the dark... kind of cool.
3 - Volume control is going to be key. Kind of like channels on your tv... some just come in louder or quieter than others so you'll want to have that handy. I've used velcro to mount mine behind the CP and can easily reach back and scroll to a more appropriate volume -- easily and without worrying about getting into the PC or some other difficult to reach quickly method.
4 - If you were my neighbor, you'd start making enemies if you didn't respect my air space and noise considerations as much as I'd respect yours. Sure, maybe you were jesting about it all, but it IS something to consider. While MAME is a blast, like all things, consider moderation.
5 - That $150 you've got for an audio budget is about 4x what I spent, including shipping, on eBay getting the Creative speakers and the Xplods. Took about 12 minutes to fully splice and dice, can take less if you don't have the issues with the creative speaker plastic that I did.
6 - The extra $100 can readily be spent on other things that will satisfy your need for all that extra sound. Like a bigger monitor. Or fancy-colored T-Molding. Or a hooker.
7 - Just my opinion. Just saying. I like "the best" for things, too, but somtimes it pays to step back and determine if it's the right move.