working with about 1.5 ft^3 and 1100 rms!

F3 is the term used for the -3 db point, where the output curve slopes downward on the low end.  -3 db is literally half the output of the reference point of the curve, and so is considered the bottom end of the frequency response  (more, and here) .  F3 is looked at when you are designing a sub enclosure and are trying different drivers, box volumes, tuning ports, etc.  The higher freqs like the 50's is great if you're building for SPL because you'll not be wasting power in lower freqs.  All the power gets transferred to the car's resonance frequencies and you can probably score higher.

But for anything linked to the term SQ (a.k.a. what you would want to listen to while driving), it wouldn't be acceptable, at least with the music that we have today.  There are a lot of vital freqs lower than 50 Hz.  SQ is a relatively flat response throughout the sub freq range so that the vehicle resonance itself doesn't play a huge role in the overall sound.  It would be the sound quality of the enclosed sub that you hear, not so much the combination of sub sound and vehicle resonance.  So you'll want the sub to perform at the lower freqs smoothly and allow those low freqs to play powerfully.

As far as I'm concerned, you have to decide whether you want to build for SPL or SQ...there isn't a both.  With a DVC 4 12" sub wired to 2 ohms, you could build for SQ.  With the rigidity that you need to build into the enclosure and the driver displacement, you should have around 1 cu ft or a little better to work with as air volume.  There are a lot of choices in subs with this criteria, and power handling should be about 500 watts.  That would open a much bigger door for you in sub selection than trying to find 2 or 3 10" subs that will give you a full range in that space.