The factory radio is 160 watts peak power, which ends up at about 64 watts RMS true useable power.
Both the front and rear speakers can be replaced with 6 1/2 in components. But you will have to build a plate to mount them onto. The factory speakers have an oversized triangle shaped mount and you will have to replicate that shape to mount the aftermatket speakers. I used two pair of Alpine SPS-171A ($200/pair) component speakers in my tC. They sound much better that the stockers but you should run a four channel amp to them because they are not as efficient as the factory ones. I would suggest an Alpine MRP-F240 [160 watt RMS, 320 watt peak] ($200) for the four corners. This amp has speaker level inputs, so you won't have to buy any special adapters to hook it up. Turn on the built in x-over to high-pass.
The front tweeter can utilize the factory tweeter bracket to reatin the proper mounting angles. Simply remove the factory tweeter from the bracket, cut off the two tabs that retain the factory tweeter in place and drill a hole dead center of the bracket. Line the Alpine tweeter up with the hole and insert mounting screw from the back side of the bracket to secure the tweeter. Done!
The rear speakers will take a little bit of creativity to mount the tweeters. I used a grill from the Alpine SPS-170A coax speakers and mounted the tweeter coaxially to the grill. If you don't want to go to the trouble of installing components in the rear, use the Alpine SPS-170A coax speakers ($100/pr). They'll work just fine for the rears.
I took pics w/ my camera phone but don't have the ability to upload pics to my puter yet. I'll post pics when I can.
I would also suggest to install Dynamat in the doors and rear side panels while you have it all torn apart. This will make a big improvement to the sound quality of the system as well as reduce road noise.
You can add a sub amp to the stock deck as well. I suggest pairing up an Alpine MRP-M350 [350 watt RMS, 700 watt peak] ($200) amp and an Alpine SBR-122CR 12" ($350) preloaded sub box. This combo rocks!! And the amp can be daisy chained from the MRP-F240's non-fading pre-out. Turn on the built-in x-over and bass boost on this amp too.
Mount one amp under each seat, don't mount them to the box. That just makes it easier for a thief to take the whole system.
If you buy both the male (radio side) and female (vehicle side) wiring harnesses, you will be able to make all connections to the amps and speakers without cutting the factory harness.
Connect the male harness speaker wires to the speaker level inputs of the MRP-F240. The male harness will also include an extra wire which will be used as the amp turn-on lead (radio has this ability, but is not used in the factory harness). Connect this wire to both amp turn-on inputs to eliminate amp turn-on pops.
Connect the female harness speaker wires to the speaker outputs of the MRP-F240.
You should also run a 4 awg power wire with a 60 amp fuse from the battery to an 8 awg distribution block for power. The fuse should be no more than 18 inches from the battery. You should also use a 4 awg cable from your battery ground terminal to chassis ground. The factory chassis ground wire is NOT sufficient for the current that the amps will draw. Ground wires on both amps should be 8 awg. Factory battery and alternator will be sufficient and you should not have to upgrade them.
This 1000+ watt system will pound. You'll be impressed, especially for the money!
These are my suggestions for "upgrading" the factory system. Considering everything else you've already done, the stock deck would be the weakest link. If you took it a step further and replaced the radio, I would suggest the Alpine CDA-9833 ($450). Using this radio would allow you to eliminate the four channel amp because it has a 104 watt RMS (240 watt peak) V-Drive amplifier built in it, saving the $200 cost of the MRP-F240 amp (although keeping the amp would be OK too). The radio features a motorized faceplate that plays both MP3 and Windows Media files. It also has Quick Search to navigate easily through your compressed media files, Media Xpander to restore the sound quality of your compressed media files, built in x-over, time correction, parametric EQ, three lines of text display, three pre-outs, XM and SIRIUS ready, I-Pod ready just to name a few of the features.
Check out all of the specs on these products at:
www.alpine-usa.com
Hope this helps with your questions. Sorry it was sooo long winded, but I wanted to be thorough.