It's actually the VF37(plain bearing) and VF36(ball bearing) that are the twin scroll turbos. The reason I said do the other stuff first is that it is still cheaper and much easier than importing and installing a twin scroll setup. Most will be satisfied with that and call it a day.
What you have to be realistic about is what a twin scroll setup will get you. For less money a larger standard turbine turbo bolted up and tuned will offer more peak hp potential. What you get with the twin scroll is about the same power potential as a stock STi VF39, with faster spool-up. If you want more peak power, you are barking up the wrong tree.
Regarding spending on US spec upgrades first, unless you import an aftermarket JDM downpipe you will be fabbing a downpipe anyways. Some who have done the twin scroll setup have simply cut the flange off their USDM aftermarket downpipes and welded on the twin scroll downpipe flange. The uppipe would be useless, but the resale market is strong for them. Heck I just bought an STi uppipe from a guy who did a twin scroll swap on his STi. So really you won't be spending the money twice. Trust me, I hate that.
Unless you've driven a twin scroll car and have to have it, I'd just pursue the cheaper proven USDM mods. Really if you think of it, swapping to stock JDM parts is the epitome of spending money twice. You paid for the car with a stock manifold, turbo, TMIC etc.. now you are spending money again for a stock manifold, turbo, TMIC etc...