You can push around 17 psi no problem with stock twins and OEM injectors (With some minor tuning). Anything after that is a bit of a risk (The ceramic turbine side will turn to dust at some point.) Other than that get some form of tuning device, eliminate Fuel Cut, get a better flowing exhaust and intercooler piping and you are pretty much maxing out what the Stock twins will do with out blowing them up right away. Consider a boost controller (A good Manual one will work), Boost gauge, and a wide band.
Oh by the way I'm selling a 3" RSR Downpipe with a wideband bung already welded on that even bolts up to the stock twins Y pipe!
Easy answer, gearing. I put a 4.10 OS Giken diff in mine, and LOVE it.
More thoughtful answer, all depends on what you're using the car for. Set goals, then modify to suit. If you're doing autocross or drifting, a fairly small turbo that favors response will be your best choice. Drag racing, road racing, quick street car, highway monster... what do you want to do with the car?
I don't regret it for a second. Had the engine been happy to rev as high as the engine was built for (balancing wise), they would have been a VERY good combination. As is, due to cam selection / setup, and turbo sizing, my engine basically calls it a day and starts dropping off just past 7k.
However... with nearly 500whp on tap, you end up shifting about as often as those Subaru guys haha!
The 4.10 in the MK2 and dog transmission seems just about perfect for 4th gear, the RPM, and the 1/4 mile. The last dyno verified speed I did at 8,800RPM in 4th, was 156mph. I didn't have a tire for it, so I calculated the 5th gear speed to 208MPH at 8,800RPM. I'll never go there.