[MA70-3.0GT]

Cool, I'll look forward to following this. It's not too often these days that someone does something really different.

I'm not sure of the differences between the Jap. & U.S. versions of the turbos but the alternative material you're thinking of could well be Nikasil, a hard coating commonly used on 2-stroke bike engines, it's that good that a couple of thou coating is put straight over an aluminium cylinder bore & it's then capable of outlasting a steel liner!!! In fact on the bike barrels, the only time it needs to be bored & recoated is if the engine siezes & the piston melts into it, they just don't wear out.
Something similar was also used on Wankel rotary engines to avoid the rotor seal/housing wear & melting problems they had (A groove was machined into the housing & a band of molybdenum alloy, I think it was known as Molykote, was used as the sealing surface along with Nikasil rotor tips.)

Either way, if your turbo has either one of these treatments I think you're onto a good thing! I take it you have the original wastegate actuator fitted to the turbo at the moment, if so then I believe the boost can be taken up from the stock setting by adding a measured shim to the assembly. Or you could just use a bleed valve to "trick" the wastegate into opening at say 15psi. The shim method is in a thread on the Mk3 area IIRC.
If you don't have the actuator, or are going to replace it then you can usually get uprated units that operate at a higher boost pressure anyway, and if you're building from scratch a new actuator would look nicer IMO (depending on how visible the turbo is in your engine bay).