Your losses, if anything, should be minimal. For the most part, bass is about pressurizing the cabin of your vehicle, and isn't completely directional. As long as you have a couple inches minimum between the drivers and the floor, you'll be fine. On the downside (pun very intended //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif), a down-firing setup will send acoustic energy outward in every direction, so you're not going to get the brute force of the energy you're creating. In some vehicles, that's good. I did a conversion van a couple years ago where we made a down-firing enclosure and that was the only thing that sounded good throughout the vehicle regardless of the seat. Also, the guy had kids and didn't want them accidentally damaging the sub, so it was a win-win all around.
I've had the most success with an all-rear-firing setup in a SUV. In that particular style of vehicle, it functions similar to one of those nasal-sounding bullhorns/PA horns, where a small diaphragm fires into a horn that reflects and amplifies the sound using a small amount of power, but it won't function as well until you put Dynamat or a similar material on your rear hatch/doors, roof, etc.
Your biggest source of parasitic loss is vibrating body panels, and the introduction of road noise while in motion. SUVs and vans suffer from this more than any other vehicle because of the large, long roof and larger body panels. Try to reinforce and/or dampen your roof, as well as dampening the rest of your vehicle. The less your car's body is moving, the less SPL you're missing out on, plus it has a much cleaner appearance when you're filling the air with sick bass and it doesn't sound like your SUV's about to rattle itself apart.
When it comes down to it, what you honestly need to do is go to the store and buy a sheet of MDF, and make a couple boxes. Start off with an enclosure that has your subs upfiring and ports rear firing. Then flip it over to see how it sounds with them downfiring (be sure to leave a few inches of clearance for your drivers to move/breathe). This is best when you have a friend around to help you out, so you can stay up front and listen while they flip your box. Then try an enclosure with everything facing the rear. If you wanna get really fancy, get an SPL meter (available at RadioShack) and run test tones through your amp from a laptop, and record the SPL levels from 20-80 in 5Hz increments. Then chart them out and see which box plays with the curve you desire.
What SUV are you putting this in?
edit: be sure that when you're testing enclosures, you have your doors shut and windows up/down or however you would keep them when you're driving. And for Pete's sake, test it in a big parking lot, not your driveway.. My next door neighbor walked all around my block trying to figure out where the bass was coming from that was rattling every single window in her house so badly because I was testing a pair of Treos on a 6kW amp in my driveway. Since then I've always done it somewhere public and away from homes.
