1)The A/R number refers to the compressor housing itself and not the wheel ...The wheel for the sr version is larger and trim is also different, so even if you machine both the comp and turbine housing of the gtr spec gtss(if possible at all) to fit the sr gtss cartridge, god alone knows how it would perfom. So to get the flow rates of the 0.6A/R housing you have to use the said housing along with its respective wheels.
2)Gtss cartridges are not available from Garrett
3)Gtr spec gtss have no HKS stamping on them. The only thing HKS about them are the actuators.
If you give us the numbers on the id tag on the cartridge we can confirm for you.
4)^^^
and i have no idea about the sr version
With that said just put the gtss on there as is....if it worked better on a rb26 with a 0.6A/R housing, hks would have given us that option.
These turbos are good up to 300-320rwkw if you want more there are the HKS2530 or equivalent Garrett -5....these are not as big as the Gtrs but put down some good power too....lots of threads here on them