Whether they get loud enough is really a matter of preference. In an infinite baffle like in a sealed box, your maximum output is determined by how much air the driver moves and the frequency playing. A pair will have the ability to get louder than most any front stage you could imagine, so in that right they will get loud. That often doesn't mean much in a vehicle. People are rarely after flat response and want much more output from the bass than the rest of the system. If that is the case, then they may not be loud enough.
If you're looking to get really loud then you generally wouldn't be looking at infinite baffle. The point of infinite baffle is to get very low distortion output with low amounts of power. To get very loud you want large drivers in big vented boxes with lots of power.
John