I know what I said, and if I was inclined to make a change that's what I'd do,
after I took out one of the stock drivers, measured its Theile-Small specs, modeled it in WinISD 0.7 (
http://www.linearteam.org/), then modeled potential replacement drivers to see if any worked better. I'm just not inclined to do so right now.
Reverse mounting isn't free air, which is no enclosure. It just aims the front of the cone into the enclosure instead of upwards toward the seat. That doesn't affect anything, other than allowing one to use a driver that's too deep for the magnet structure to fit into the enclosure. BMW and direct replacements get around that by putting the magnet in front of the cone instead of behind it. That works, but most are outrageously priced, and as I've said none have conclusive data to prove their worth.
If you're the curious sort and want to try it just to see what happens the good news is that very good eight inch woofers can be had for $60 or less. One that might be worth trying is
https://www.parts-express.com/dayton...4-ohm--295-376
I don't know if it will fit, you'd have to compare its measurements to the space under the seat. It's a bit of a pig in a poke, but since just about every direct drop-in doesn't post any real specs they are as well.