Interesting thread. I've owned a BA XR8 and loved it. Now I own an Mk2 GT and the difference between to two is huge. On paper it may not sound that much maybe but let’s have a look at it anyway.
The GT has a stronger, more powerful engine –higher compression, different pistons and cams etc (value over XR8- maybe 3k),
18" wheels standard (value over XR8- maybe 1.5k),
Premium sound and climate control and much nicer interior (I have leather)(value over XR8- maybe 3-4k)
Bigger brakes (value over XR8- maybe 2k),
Stiffer suspension (value over XR8- maybe .5k),
Better looking and integrated body kit (value over XR8- maybe 2k),
plus a handful of minor enhancements like stripes, colour coded engine covers, gauge pods and stuff like that (value over XR8- maybe 2k), ...
None of this is taking into account that the GT is in fact a GT, something the XR8 can never be. So if we have a look at this conservative estimate the GT has approximately between $14-15k worth of value over an XR8. The XR8 retails for $51k while the GT $61k. Some may not value the difference of the two cars but I think this highlights that the GT is hardly a rip off.
To be honest coming from a bloke that has owned both, if you think theirs bugger all difference in the two then I would assume you have not owned or spent reasonable amount of time in both.

They are poles apart IMO and you can definitely feel that extra 30kW's over the XR.