There is a performance impact moving from R5 -> R6 due to additional reads/writes on the backend, if you have the ninjastars tool it'll show you the estimates for your config. If you aren't pushing the array hard atm you might not notice it.
As you say going from 7+1 to 14+2 uses the same amount of space for parity so shouldn't lose space once complete, it will need additional space for the conversion though so keep that in mind.
I'm not too sure of the math but seem to remember a performance hit relating to larger set sizes, in the past 3PAR was normally optimised for 3+1 which was considered the sweet spot for performance vs space. I'm not certain how that factors in on latest hardware/software versions and the use of SSD though as I've not seen any graphs/details on this since the HPE purchase (we used to get a lot more low level geeky stuff out of 3PAR when they were small and you could have dinner with the senior dev/tech people occasionally).