Reddit reviews Saab 900 16 Valve Official Service Manual: 1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993

Honestly the best thing you can do is getting the car to run at OEM spec, then think about upgrades. A stock 900T should boost about 10psi or right about the yellow/red line on the stock boost gauge. A real boost gauge is very helpful in troubleshooting this, and while you're at read up on how to check and adjust the base boost setting of the wastegate as sometimes these get tired as well.

If its not doing that it can be variety of things, even if the rest of the car has been well maintained the engine sensors may be 25 years old at this point and wear out. A c900 is very picky in my experience about ignition components, so make sure you have the correct spark plugs and it can't hurt to replace the distributor cap and leads. Check for vacuum leaks in the engine bay, and get a spare air mass meter as these are finicky as well.

After it is up to proper spec you can adjust the APC to SPG spec (~12psi) for a nice little HP bump, or even a bit more. Google 'Saab apc mods' or go to 900aero.com to read up on how to do that.

If that is still not enough power look into doing a Trionic 5 conversion on it (engine management system out of a 9000) as IMO it is the best mod you can do to a c900 to get it to perform better.

Theres no harm in shifting after 4000rpm but the Mitsubishi TE05 turbo your car should have really runs out of steam around that rpm, and quickly heatsoaks the intercooler, so theres not a ton of power to be had as you get further up redline.

Other than that as has been mentioned realize these transmissions are fairly fragile so don't go around dropping the clutch at redlights, slamming into gears etc. Just accept that these are a fun, quick hatchback from the 80's and most minivans these days are faster ;)

Oh and buy a Bentley repair manual:

https://www.amazon.com/Saab-Valve-Official-Service-Manual/dp/083761693X