Having had the 4200CC Facelift with upgrades, it was close to a GS..!! One difference I did notice very much with the GS was high speed stability, it's excellent compared to the slightly skittish drive of the 4200 at speed..!! The GS at 170mph is rock steady and maybe something to do with the additional body mouldings which must work..!! The 4200 is still great though, but needs xenon lights..!!
My 2007 Facelift has the xenons and the throttle blip too. Other than valved exhaust then there isnt that much between the gs and late 4200's running boards and interior trim aside.
Off topic I know but if high speed stability is a problem then it's definitely worth checking the front-rear rake angle. Quite small adjustments, probably not visible at a glance, can make a really big difference.
My 02 Spyder has Xenons as well, reversing sensors etc. Definitely glad of both.