I agree with James and Rock. Just do it the first time with 8 guage, so you won't have to do double your work. You can get a small roll of 8 guage. I couldn't tell you exactly how much, cause it will depend on how you travel the wire. Like at first I was going to route it right under the driver's side like everyone else said, but then I ended up drilling a hole in the passenger side like Rock had done, and it saved me couple feet.
It's just helping to give your amp a longer life, because it's not starving the amp of power when it needs it. It's really hard to say if 8 is totally necessary, since you say you don't plan on running them at full blast, but full blast to one person is different from another.
If the amp is running 250 watts RMS from bridging, then that's 250 (subs) + 75 (right) + 75 (left), about 400 watts RMS. I know you're just using Pioneers at the moment, but someday you might want to get others that take more power. Try also, to first run your amp with as little gain as possible. The way to setup an amp is to turn the gain down and turn the head unit all the way up till you hear distortion. Turn it down till you don't hear it anymore, then turn up the gains secondly till it distorts, or it's too loud for you.
I don't think 8 guage kits are that much more expensive. I'd say take the one you got back. They should be no more that $30, and that's on the pricey side. For some reason, some kits come with RCA's others don't so make sure not to just pick one up without looking. There's not going to be any major differences at this level between manufacturers, so you could even go with a lower name brand. The only major difference will be in the RCA's and most people can get away with plain ones if they route everything correctly to avoid hiss, ground loops, etc.
Good luck,
Jon
