Unless you absolutely positively have to have the tour pak speakers now, you may be better off waiting a little. The radio flashes seem to be wreaking havoc, and no one has a solid answer for it yet. I'm sure it will get taken care of, but it could be mighty frustrating until it does. Just adding 2 speakers may not be an issue, but you never know with the moco.
You are very correct in that driving the rears off the HU will be very underpowered (read that as you'll probably be disappointed a month down the road). I have a 14 tour pak on my 13 RG and am waiting for BT to come up with pods that will allow the 7.1s in the back. The biggest thing that fits in the stock or aftermarket pods that I'm currently aware of is 6.5 speakers. BT says their new pods will be available over the next couple/few months.
I did my stereo upgrades in steps and smack myself for it every time I think about it. Just about anything will sound better than stock, but without a decent amp driving them, everything will seem kind of blah after awhile. Big difference in the garage, but you'll quickly realize it's not a huge improvement going down the road and want more. I started out upgrading from the stock speakers, then added an amp later, and finally bit the bullet and bought a full system. Would have saved some $$$ if I just bought the full top of the line system first. Do your homework, there are some very overpriced and underperforming systems out there.
Freedom isn't free, remember to thank a vet.
2018 CVO RG lava orange, 117 Stage 3, 14" F47 and a few chrome extras. Boom crap gone.
2016 RG Special Cosmic Blue, 120ST with Dragos 2:1 and TTS tuner, FBI drop seat kit and JNR air ride. Stock audio long gone.