tsenfw 10+ year member
CarAudio.com Elite
+1 yeah what he said. The Q is FI's sound quality oriented line of product. Still gets ridiculous loud though while sounding good. Those rms ratings (when getting subs from a REAL submaker) are set to be really conservative. I think the general consensus is 1200watts sounds good on the 12" Q's without even beating on the sub. So that kinda shows the kind of power they can take.the hdc315 needs more power to get going. So off the amp you have i would say the fi q is your better choice. If you had more power i would lean to the hdc315.
If you measure the space you have while still keeping access to back seat I'll help you figure out if it's possible to fit. Big difference in box size between ported and sealed. FI recommends 3-4 cu ft for a ported box, that's pretty big and heavy! lol
https://ssl.perfora.net/www.ficaraudio.com/sess/utn;jsessionid=1547a12326ba58b/shopdata/0020_Q/product_overview.shopscript
So go measure before you decide on anything.
You'll probably want it to fire torwards the rear also. Said friends 15" Q in a 3.0 cu ft sealed box we threw together. Sorry for the phone camera quality haha