You cannot match the HSK ARs, as HKS has proprietry on them.
I am pretty sure Ben Ellis is using a Gt3040 on his GTT in HPI mag but with a 1.** turbine AR, also I think GTSt VSPEC just fitted one.
The hks 3040 uses 50 trim comp wheel, the 56 wheel is used on the 3037 by HKS? the turbine trim is the same.
HKS sell the 0.87 A/R turbine housing with the 3037 as a kit for the RB25, I currently have the 0.61 and although it makes good power at 1-1.2bar, I am sure it is going to choke the engine a bit up top once I start winding in boost, also it is only making 256 at 1bar, so probably a bit more with a larger AR on the turbine, which I intend doing soon:)
I think your choice boils down to where you want the boost and how much power you. The bigger the AR ratio the better the top end, more ign, more power for the same boost pressure - due to the reduced back pressure (the curse of turbo powered engine) but also more lag, although this can be offsett by the use of adj cam gears or cams.
Also if you intend doing cams you will get increased overlap, so a smaller AR could really cause troubles.