You can't expect the subs, to act as supplemental propulsion units when the car is moving, and the trunk is open, now would you?
Seriously, your only enemy is weight, and too much current draw. Get a separate dedicated battery for your system, a larger capacity alternator, thick gauge wiring for your new battery (preferably also get a deep cycle one) and a large enough fuse for your main line. As you can see, all that adds weight...
additional weight sa car. amps, subs and box minsan mat deadening pa. Kinda like having an extra person sitting in the car at all times.
Bigger concern is the electrical load. You may experience dimming lights, drained battery (if you listen with the engine off for long periods of time), static etc...