Not necessarily, it all depends on what you want from your system. And what kind of music you listen too. If you are going for SPL, then yes 8s are probably a waste of money, but if space is a concern, and you are more concerned with SQ then SPL 8s may be just the ticket. I replaced the mazdaspeed stock 8" subwoofer with a JL audio 8" subwoofer and it's much better then the kenwood sub. It's no 10" or 12", but you can't expect miracles from a small speaker. But mulitple 8s in a small enclosure, that has potential.
I would only get JL audio 8's or the edesign 8s (which happen to look very nice) Obviously if I can go with 10s and still keep my trunk space to a reasonable amount I will. We'll see how things go after I talk to my installer and see what we can fabricate with my goals in mind. But I have a feeling that multi 8s 3/4 will fit my needs nicer then a single 10.
The big problem with the MSP/P5 is the lack of space in the spare tire well because of the hump. That is what I want to look at first. That's why I am looking at multi 8s. If I can't do something in there and keep the floor low enough, I'll look at other areas of trunk and all other speaker sizes.
okay, it's 3am, time to quite rambling and to bed.