OK, another "I don't want to add too much weight" question...
So do you plan to race your $30K+ pocket rocket? You plan to strip it? I really don't understand the concern I see around the net dealing with the adding weight issue for a streetcar. I have seen this question asked several times. I say:
If you are serious about sound them put a serious sound system in it!
If you are serious about racing then make it a serious racecar!
If you want to do both, be creative!
Why not install a nice basic SQ system that uses quick release harnesses/connectors/bolts/etc. This will allow you to pull the amps and subs on race day very quickly...Besides, if you are that serious about racing you will be tuning the car before the race anyway, right?
I see no reason that you should not be able to install a set of components, a five channel amp, and a single 10" or 12" How much weight is that? Mitsubishi even offers an upgraded system with a trunk-mounted sub.
Just buy a bolt-on HP mod and that will make up for the extra pounds?
Here is a nice but simple system that will allow you to take advantage of the 880PRS in the 3-way FULL ACTIVE network mode and should have great SQ:
Amp - ECLIPSE XA5000 5 channel -
http://www.ebay.com
Mids - DAYTON AUDIO RS180 (Looks like you have 3 inches of depth and a large opening.Was told that most 6.5 inchers need an adapter. The Dayton’s have a 7" frame and with a 3" depth so you should be right there with a little caulk spacer. If you have not heard about the Dayton’s, they are very nice mids. Do a search) -
http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=295-374
Tweeters - ALPINE XT19 - These are VERY nice VIFA based tweeters from the ALPINE SPX-177R comp set and are very small and should fit behind the stock A-pillar grills. Also, these play low(2kHz) which allows them to mate up with the Dayton mids that can only play up to about that point. Can buy them from PacParts for very cheap.
http://www.pacparts.com/model.cfm?CFID=767269&CFTOKEN=85991391&mfg=ALPINE&model_id=SPX177R&row_start=1&src_model_id=spx&action=list_part
Sub - DAYTON AUDIO REFERENCE SERIES HO 10"
http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=295-462
Review:
http://www.diymobileaudio.com/forum/showthread.php?t=1953
OR
ELEMENTAL DESIGNS 110v.2 (Dual 4Ohm)10"
http://www.edesignaudio.com/edv2/product_info.php?cPath=21&products_id=34
Can get one for $120($55 off!) if you join their forum at
http://www.icixsound.com then shop from here:
http://www.edesignaudio.com/edv2/advanced_search_result.php?keywords=forum
Both of these subs will run in a small box(~.5 cu ft)
Deck power is alright for very modest power to the front but won't do a whole lot for the rear 6.5" subs....But why stop there with a great all in one deck like the 880? Will get much more detail and impact with just a single 5 ch and a small 10" sub. And you can remove it on race day!
If you insist on using the deck for subs, at least get an amp for the system. And use the eD 7Kv.2 subs instead, much more capable:
http://www.edesignaudio.com/edv2/product_info.php?cPath=21&products_id=33