Two serious improvement firms in Europe have special enhancements for Maserati cars, one is in germany (DIMEX Group) which has the LeoNardo Maserati tuning.
I know they do new cars as well as tuning your existing car (3200 and 4200 Coupes), they have 4 stages which goes to 450 bhp and even a turbocharged 530 bhp!
Apart from engine tuning, they also offer a wide range of body upgrades, from wheels, brakes, suspension & springs, exhausts, aluminium parts, carpets, steering wheels, sport seats and loads of carbon items for the interior.
The LeoNardo Maserati cars i've seen are really impressive, but not my kind of thing, i personally like to keep the car as-is from the manufacturer.