Wouldn't recommend it. What polyfill really does is reduce mess with the impedence and will really just make you lose output at tuning. Polyfill will mostly lower the extra output your getting at tuning. They use it in vented boxes sometimes to reduce the upper harmonics coming from the port, but a PR setup doesn't have that issue at all.
You could try a sealed box with just one woofer, but I don't think you'll find it better. Part of why people like sealed boxes (when they do) is because most woofers have crappy upper end extension and if you push the total Q of the enclosure high enough, it gains a bit up top and loses most of it's low end, sounding half way balanced lol. Not the best way to do it, but it is what it is, I suppose. You'd want to keep the higher Q fi Q if you did that, as it would give you the most top end for the money.
Anyway if you'd like to try one more fairly cheap suggestion, I do have one. Try out a dayton HO 12 in the same box. The reason your Fi sounds muddy may not be due to it's tuning. It's because Fi doesn't know how to properly use shorting rings and all their drivers sound like crap on upper end bass notes. Check out the review on http://www.data-bass.com for the Q as well as the SSA XCON (same build house) The Dayton will give you the same response in the that box (minus inductance effects I can't model) and has very good inductance. It won't be quite as loud due to the reduced powerhandling, but will sound probably sound more balanced than your current subwoofer. Dayton underrates their stuff anyway, I'm sure it will handle more than the 400 it's rated for by a good margin. If it was a car audio driver I'd bet most companies would rate it around 600-800rms and the xmax is likely underrated quite a bit as well. So basically imagine your current woofer, with the volume turned down like one notch from max, with the same balance from 50-25hz, but a much stronger upper bass from 60-80hz, as in louder than what you can get now period up top... Should balance it out nicely. For the price, really any price Dayton makes very good woofers and really will compete well with most companies mid tier offering, at a bottom tier price point.
Anyway the BEST overall idea I could give you would be going up to a 15inch subwoofer and picking up one more passive. A Dayton HO15 would provide more overall bass and a much flatter response in 2.5 cubic feet. As I said, 2.5 cubes for a 12 will always be a bit bottom heavy and if that's not really your thing, the 15 might provide the overall best sound for you. That would probably make you swear off sealed enclosures for good, srs. Your enclosure is 15 tall, I don't think you have quite enough space for that though. You said you had 17inches to play with in your trunk, 16inches tall is enough to fit a 15 flush mounted, if you ever did a different box.
Anyway here is the link for they dayton 12 if you want to check it out. Like I said, won't be quite as loud, but probably you will probably still view it as an upgrade when it's all said and done. http://www.parts-express.com/dayton-audio-rss315ho-4-12-reference-ho-subwoofer-4-ohm--295-466
edit: Your box is now tuned to 30hz since you were asking.. If you want to test that, go get some test tones all recorded at the same volume and play them and watch your cone movement. The note your speaker cone moves the least on is your tuning frequency. Also, your passives will move the most, but that's not always as easy to see, just look at your active driver.
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