WOW what a mess, let me try and clear this up, I am not confused.
First the new 3 bar sensors on a stock car, no they have no benefit unless you get the tune to go with it, this can be a GMPP tune a Werks tune a Trifeca tune or a Westers tune to name a few, the GMPP will most likely give you the least amount of gains. The stock sensors are 2.75 bar and in practical application will only allow a constant 25 psi, the new ones are 3 bar and will allow about 28 with the correct tune. The sensors are not the key, the tune is. Westers tune with the stock sensors makes more horsepower and torque than the GMPP kit, eventually his tune with the new sensors will certainly make more horsepower as he will be able to achieve more boost.
Selling a GMPP, no the dealer can't help you. You can however copy the tune with HP Tuners and put it on another car that has the sensors and it will work fine as long as it is the same model and year and transmission IE another 2007 Sky , you could not put it on a 2008 Sky or any Solstice. No the GMPP tune is not mated to the VIN, the only reason the dealer uses the VIN is to download the correct tune from GM as the VIN has all the relevant info in it, make, model and year, manual or auto. If you sold a GMPP setup that had been installed, there would be no warranty and no dealer support but it would work if you had HP Tuners and someone had the same make and model and year car, and was stupid enough to buy it.
If you bought a GMPP tune, and you want more, many of us warned you don't do it, I know I did, so either be happy with what you have or pay someone to modify your GMPP tune. Right now the Wester's modified GMPP is OK, needs work but that is what winter is for. I have heard Trifeca has had good results modifying the GMPP as have Werks.
If you already have a tune, like I do, you can indeed buy the sensors, like I did, and get whoever tuned your car to make you a tune to work with the new sensors, like I am going to do in the spring.
Whew I hope that clears some things up.