Agreed the changeover from 3.9 Hotwire to 4.0 GEMS was a very blurred border.
When I was looking for my +8 in late 2002 I was offered an ex-Factory car that was said to be "the first of the GEMS" , this was a bright yellow car with black upholstery and wire wheels. I liked the colour but not the wheels and the general condition was not particularly good, so I passed. However it was claimed that car was built in early 2002. I therefore assumed (dangerous I know) that 2002 was the start of the GEMS cars.
However since then I have come across the following:-
At least 2 cars built in 2003 were Hotwire, but most were GEMS
Most cars built in 2002 seems to be GEMS, but there were several that appear to be Hotwire, especially from the early months
Most 2001 cars appeared to be Hotwire, but with some GEMS appearing from late in the year.
This makes for a very indistinct boundary.