Mike,
From my research when I was hunting for a 4200, I found various references to the upgrades made to the 4200 during the late 2003/4 production runs, when the facelift actually came out. They are out there for all to see, I'm not making it up. I think most of the changes were electronic, but there were other bits too, plus the obvious cosmetic changes. I will try and hunt these facts down again. This is what made the facelift virtually the same as the GS mechanically.
Taken from EVO: Maserati's list of significant changes for the '03 Spyder and Coupe is short but focused. It includes new software for the Cambiocorsa transmission that prioritises engine torque rather than revs as a parameter, new software and dampers for the adaptive damping system, an all-new stability program called MSP, a new steering box with more direct gearing, larger anti-roll bars, grippier tyres and a bodyshell 20 per cent stiffer in torsion for the Spyder.
Cheers
R