[Mr.ac]

I have a single 10" in a .75 sealed box, 200 watt. That's enough to rattle the mirror and trunk lid.

Honestly all depends on the enclosure and power level.
A high powered 8" in a good enclosure will sound just as good as a 12".

The problem with putting stuff in the wheel well is the amount of panel rattle. Sure it's going to take some tricks and tips to get it nice and quite.

Anything over 200 watts will do the job.