I think that Brompton and Birdy have both sourced high-end IHG with a narrower hub spacing, suitable for compact folders. These hubs are supplied OEM (to the bike factory) not to bike shops. I use Alfine on my sort-of folding Dahon and it works really well.
Shimano gears should work well no matter how much or little you pay for them. There is no need to go to XT level just to get gears that index well. Learn how to adjust the gears, the difference between adjusting cable tension and the derailleur end-stops. parktools.com is a good source.