loud as piss, but not a hell of alot louder than a properly done car could be, with ALOT less equipment that sounds better to boot. Why go for just being loud and sounding shitty (even you've said it sounds "good enough for me", but never it sounded "great") when you can have a system that will get 90% of the volume, with tonal accuracy, across ALL frequencies?
People using single 12's can put out 150s whereas multiple sub setups can hit 140's, it's all in the install. Your speakers aren't going to be in optimal enclosure, nor will they be crossed over properly. You're comromising your overall install and hoping sheer brute force will give you the volume you want. It probably will, but it's sure as heck not the best way to go about it.