How to cut down boost threshold?

If you have the stock .63 turbine housing, you can swap it for a .48 housing (nothing else has to be changed) and it'll spool sooner; HOWEVER, you'll also lose the top end power (above 4500 RPM). Always a trade off.

3" exhaust, from the turbo to the bumper will help tremendously. So will ditching the airbox and clamping a filter right onto the VAM. That's what mine is (but mine's a 93 coupe with an SVO engine), 3" exhaust (including the down elbow) and open air filter with no air box. I am seeing 5 psi at 1400-1500 RPM, 6 by 2000, 10 by 2200, by 2500 about 12, and from there it just rockets up to max, which in my case is 18 psi. If I'm running down the road at 2500 RPM and hit the throttle there is very little lag, maybe 1/2 second at the most. The downside to this setup is that the engine (and EEC) isn't really designed to boost this soon and this hard, so you will have to run 93 octane if you have it available, whatever the highest octane you can find.

The wastegate is only going to let you make so much boost BUT you can fool it by using an adjustable bleed-off style manual boost controller. Used to be that Gillis had the hot setup but he's changed his design and I dont' care for it. Try Stinger. I don't have one of Stinger's valves but might pick one up if funds allow. Still using the gillis; and it's at, as said, 18 psi-and that's plenty to annhiliate the rear tires in 2nd gear with 3.45 rear, particularly when it's cold outside. Makes it fun. It hits hard and fast if you nail it from just of idle, it will run up to about 2500 or so and then its like a light switch. There's a way around the "light switch" too, by means of an electronic programmable boost controller but they aren't cheap. The way that they work is that the boost line from the turbo is connected to the controller which bleeds off some air. This way, the wastegate still only "sees" 12 psi, even though the engine is actually seeing more, which is whatever you set it to. My old 84 SVO was a daily driver when it wasn't broken and I had it running 22 psi; but as said it had 93 octane in it full time.

Speaking of being broke, being a turbo 2.3 (old style), make sure your automotive repair skills are good....and keep the jack stands handy.