I have a 2003 MCS. I have not driven the GP but I understand it is very well balanced and handles great out of the box. I have a few mods and my car is reasonably quick. It has Jan's head so the upper end performance is strong.

Addressing only the power issue, a mildly modded MCS with Jan's (RMW's) head makes significantly more power than the MiniGP. However, what gets you moving is torque, and the 07's clearly dominate in that arena. There was a dyno day of SCMM members. Included were Minis in various states of tune, including a MiniGP. The results are posted in this thread:

http://www.clubscmm.com/board/index.php?topic=4190.135

They offer a window into the wide variation in output among Minis. The mods are listed, so the table makes interesting reading.

 

 

Sorry, I forgot about that. Recapping, GP did 187 hp/159 tq., a few hp more than the JCW's. Mine, with header, catback, 15%, alta intake, non-power generating old MTH tune and RMW head did 207 hp./164 tq. (stock injectors). An MCS with all the M7/"cosworth" stuff on it incl. schrick cam did 213 hp/168 tq. An 07 MCS stock did 179.4 hp/200 tq.