Congratulations John, excellent choise!
The GT3 CS is my second solution. The only thing I´m afraid in this case is that I would modify the car to get it closer to the RS and spend lots of wasted money.
With the RS there is nothing to improove. But the GT3 CS looks better than the RS for sure.
For the begining, I think you could replace the rear wing with the carbon one from the RS, which looks awesome! And then just change the setup of the standard suspension, lower it by 1cm (as the RS) and diferent geometry. That is what I would do and I don´t thing that it will cost much.
Then there are many mods to do to get it closer to the RS but I suppose it is a lot of money for small results.
Carbon fiber hood
Carbon side mirrors (just for better look)
Plastic rear window
Carbon fiber front bumper
Carbon fiber CUP doors
Maybe an aftermarket suspension for track use
I wouldn´t do anything with the engine, although all the GT3´s are dynoed 370-380PS and RS 395-405PS.
I hope to sell my car soon so that I can join the GT3 CS or RS club soon!
Enjoy your CS
P.S. Your CS came with Michelin CUP tyres? I thought the CS has standard road tyres and the RS has the Corsas.