The GT V6 is very fast in its standard form. There really is no need for a remap. The Q2 diff is arguably needed, as the standard one has a reputation for breaking and taking out the gearbox with it. If you were going to throw money at modifications to a GT, I would look at the suspension, as the factory fit stuff is pretty hard. But it's debatable what you should fit and this subject is the source of many threads on this forum.
Also be aware that the road tax on an 05 3.2 GT is about half that of a later model. Check the DVLA site for the exact changeover dates before buying anything!
To answer the original question, yes I would say £9K is too much for a 2005 GT, no matter how good it is.