The turbo's used in the maserati bi-turbo's are the IHI RHB51/RHB52 V-M3/V-M4/V-M5.
I couldnt find any info on their trim sizes though.

 

The turbos in the Maserati are very small:
http://www.mx6.com/forums/showthread.php?t=125826 and if you want to upgrade them I can swap two VJ11compressor wheels and housings or two Warner Ishi (T Bird) housings and wheels for you. I'd keep the existing turbine and you'd get larger compressor wheels with larger A/R housings.

For the price that I'd charge, I somehow highly doubt you'd want to do it. My suggestion is to get a boost controller and do what everyone else does, send it to the junkyard.