Chief is right, if he's doing SPL comps, thermal rating isn't what you need to consider, you'll need to consider what amount of power he'll need to get the sub to mechanical excursion at tuning. He'll need a big ported or bandpass and will need to tune to the resonant frequency of the cabin. That being said, I wouldn't doubt each sub needing 4000-5000 each, it's just about the box. If he's looking for a daily driver, the MT isn't the sub for him, look at the X.X.X. for daily driving, the MT is strictly SPL, has no warranty, and SQ isn't going to be their priority.
You'll also need a bandpass crossover so you don't destroy the sub above or below tuning, by the way. It totally depends on how much money he wants to spend, US amps make great high power amps, some are stable to .5 ohm, the Zapco 4kw is also a consideration if he's made of money.
there really isnt a limit on watts with these subs since there spl orientated, and i wouldent bother getting 2 unless hes devoting his entire vehicle to a box, i suggest 1 18 in a BIG eighth order bandpass if hes competing, that would be hella loud, but sq would be disgusting, X.X.X are already pretty much too loud for daily drivers, when i play rap music i have to turn it down because i cant even see, so to tell you the truth there really is no purpose in getting one of those monsters because it will sound like an all day burp and you probably couldent drive with one of those, it would shake your car to peices;)
ok guys ill tell him what you guys said and i had no idea they needed that much power besides going down to one 18 would save him 500 bucks lol and if he went down to the x x x hed save like 600 bucks and as far as equipment this guys got enough sitting around enough for an 18 wheeler id say and im not kidding hehe (the guy has a pheonix gold reactor he paid 2400 bucks for only 500 made waiting to go into a car as well) personally i think that amp was a big waste in money for what made it up but at least i got to see a rare amp lol