1) 5.8psi is still too much.
2)The Tial 38mm will easily vent enough exhaust gasses to keep you at 5.8psi. External wastegates are the king of controlling boost spike. But since you have an automatic, you may still run into issues with spiking. I believe Bigwoofer blew his motor because of this...
3)The GT28RS still spools slower than the stock MSP GT25R turbo. The GT28RS However more than makes up with it in the top end. Also remember that the stock msp turbo has a .64ar turbine housing, while the GT28RS has a .86ar turbine housing. The larger the turbine housing, the more lag you will experience, but with larger turbing housing your peak HP number rises too. The turbo you have will not spool as fast the gt28rs, because of its massive compressor housing. I dont think you realixe just how bid it is. A GT40 Compressor housing is no joke. Keeping things simple, the turbine and compressor housings are matched together, to create a create a turbo thats serve's different applications. There are a lot of variables that play into spool time (time, wheels, ar, etc etc) a good general way to look at it, is that the larger the comp wheel, usually means a slower spooling turbo as well.
4) I honestly can't say for sure what will happen if you use. Running at 6psi, im sure your block wont last long at all. At 3psi, you might be OK if you can keep the car from spiking at all. Honestly, I don't think it is something you should gamble unless you a) have another car you can drive if this motor blows and b) you have the money to buy a new bottom end, forged internals, and possibly other parts that are damaged when the engine gives out.
Not exactly. I really don't know why these hybrid turbos are such a hit. They are made to fit a very specific purpose or application. Yes, it will really slow it down that much. And come to think of it, im sure this turbo will have so much compressor surge (not turkey), you'll get tired of very quickly and want to get rid of it