Agree with the above but would also recommend you test ride several other bikes from other manufacturers in the same category.
The frame geometry will differ between bikes and will contribute to the general 'how it feels' when you ride it factor. Its one thing having the best spec you can afford but it still needs to meet your needs - the GT sounds like a very good benchmark, but try Trek, Kona, Specialised, Marin - see how they compare.
You're right about the weight consideration though - stick to your budget and get the lightest, best spec'd you can afford that feels right.
May have to compromise a bit though to get a good balance between road and downhill capabilities - a bike set up for each of these applications will differ IMO