I went with Focals in the front and back. New 12" sub in the trunk and two Kicker amps. I did however replace the stock HU. I do suggest you consider replacing it. I don't believe sync will be that hard to reinstall. Just don't want to spend all that money on speakers and under, or over power them with the stock HU.
Which dash kit did you end up with, got any pics? and I am assuming it would require some fancy magic wiring harness to retain Sync and the steering wheel controls if possible, and likely only compatible with certain models of aftermarket HU as well.....yes I know I am a cynic but I have it in the back of my head that all the extras to do a HU correctly would push the cost of the total HU package past 1K.
13 GT base, 3.55, Sterling Grey, MGW shifter, BMR LCA's, GT500 AB, silver RTR's, 275/40/19 Michelin pilot super sports, more stereo equipment than I will admit to, more parts to come.