hey dezz, i know your posts are a couple weeks old but just thought id post on the chance that you'll check this before you spend your hard earned cash. I do work for JB but in qld so please enjoy my humble unbiased opinion.
I read through all you guys posts and was glad to see you get a lot of good advice from everyone such as the importance of SD and some good gear recommendations, however, please understand its great to shop off the net and educate yourself on specs and prices but please do yourself the favour of checking a few shops and listening to the splits on offer before you commit to anything. Also if i can give you one piece of advice to hang onto its that everyone has very different opinions on the exact same matters because everyone has different tastes and preferences and you need to work out yours rather than listen to the person who sounds the most confident about what is the best gear around.
Firstly i appreciate you wanting to keep it all a uniform brand and pioneer at that but for what its worth and again just my personal opinion but to put my opinion bluntly pioneer decks are fantastic, pioneer splits are great, pioneer subs are nice and pioneer amps are fairly nice. Nothing pioneer sells is rubbish but please remember there is a lot of awesome brands that do awesome products to compare the pioneer stuff too.
My biggest concern with the setup you have your eye on is that the TSW306C is going to be far from impressive running off the back channels of the GM6100. These subs can be made to sound good but not with any less that 300wRMS into them and preferable closer to 400rms. As in get a seperate amp and part with $300-400+ for anything resembling a decent result.
Okay i'm probably boring everyone a bit but just a quick word on the no back speakers issue. More speakers in a car will always sound like a good idea in theory but very rarely true. Imagine just using a four channel amp like the one you have mentioned for front and rear speakers and you have a car with a fair bit of noise in it. Preferable option for myself personally would be bridge the front and rear channels of the four channel to run just the front two speakers and forget about rears altogether to get the absolute best out of the speakers that matter the most . "Imaging" got mentioned a few posts back and to clarify this basically means being able to sit in the front seats of your car, shut your eyes and be able to distinguish where the different players of a band for example are standing on the stage in front of you. This can also be enhanced by getting a deck with time alignment or time correction such as the DEHP-8650 to centralise your left and right channels around your head like your sitting in the middle of the car.
Might sound a bit pointless with the sub sitting behind you anyway but remember the bass from a sub or subs has no real direction and in properly tuned systems it is very hard to shut your eyes and point to where the sub is located because it will just reflect off the front windscreen and help to establish the imaginary stage on your dashboard if you are just running front speakers. With the money you save on buying rear speakers and running a decent amp for them you can spend a few extra dollars on your front splits or your speaker amp. Also you mentioned just running your rear speakers off the deck which is can be done but basically means you will have to fade your sound to the front speakers every time you want to push your system hard anyway or tolerate the harsh and distorted sound screaming from the back of your car.
Okay wrote way too much sorry if that was painful to read but hope it helps you out a bit and if you have any more questions or want some gear recommendations i'll be checking back.