You're already looking into changing the sub? Hifonics are damn cool man I love them, courtesy of Sumil Lol. Some guy I know is offering me a 12" FI SSD or something sub for around £180 barely used! That's a lot cheaper than a 12w7 so it'll be interesting to see which route I eventually take but very keen to hear an FI / RE compared to JL w7.
If you could fire thr sub through the skit hatch / rear arm rest, into the cabin, the SQ you get will be outstanding! The other thing you could do is dynamat the boot lid.
Now: E92 335D
Gone But Not Forgotten: E90 320D Alpine White