Anything that bolts up to the stock dump pipe is a compromise for the 60mm turbine. There has been disappointment from these housings in terms of their power they make at the top end. So while you feel like you are saving some money on the front end you may end up with an expensive lesson if the results aren't what you want.
The GTX3071 'on paper' will make similar power to the older 76mm GT compressor but history has shown that the AVO will probably max out before the compressor has done its best.
The ARR is just what it sounds like and area radius ratio so there is not much joy in comparing the 0.73 to a garrett 0.63 or 0.82 GT30 framed ARR. The 0.73 seems to flow less than the 0.63 (The Mafia has done more to max out a 0.63 than anyone if you search for it).
I think you will get better results if you suck it up and change dump pipes and use a native garrett housing.
Hypergear results recently are showing some really good results that are very similar on paper to HKS2835ProS, plus the Kando Dynamic turbos also are producing good numbers at great prices but also require a little bit of fiddling around, like a Garrett turbo, the first time you install one.
If you are committed already I would ask if you can change to the Garrett turbine housing instead.