Wanting input on my "setup"
 

I'm pretty new to the car audio thing. I'm only 21 and decided my stock ford 6x8 speakers aren't that great. They weren't the Mach 460.
I replaced the head unit with a pioneer 50x4
I installed 4 infinity ref 8602's
And a infinity kappa 5 amp, along with a infinity kappa 12inch sub in the trunk with an enclosed box. The wiring is stock wires to the speakers. My head unit powers the front speakers and the amp powers the rear and sub. Sound is decent. It's loud and the bass is somewhat there.
Should I tear out all the factory wires and add new wires? And I'm wanting to downsize my sub to either a 10 or two 8's. Since I listen to mainly metal. Slayer, anthrax, ect. I hear 10s kick better than a 12. Plus my 12 hardly fits in the trunk. What do you think of my speakers and amp? What are other good brands out there?

smaller woofers hitting notes faster or more responsive is a myth. It comes down to box tuning mostly.

If you were to build a ported box thats tuned properly for what you have then it will only cost you $40 or so and you'll have more output aswell as more response for next to no cost.

The downfall is space. Ported enclosures will generally be larger than a sealed enclosure so if you're concerned with trunk space then this may be an issue. But what do you really want??? A good sounding setup ? or something thats hidden away nicely to leave lots of trunk space?? Without spending $$ you can only have one or the other.

Well, trunk space is very limited. I have a 94 mustang convertible. Had to make my box smaller than 1.2cft to even fit.
So I know it's not proper size. But I'm making due with what I have.
I'd like a sub that can play double bass well. You know like kick drums and stuff like that. I'd really wanna have a ten or two eights if they would be better considering my space.