What is not XT on the Pathlite 7 are: bottom bracket, cassette, chain, pedals. The crankset is XT.
Bottom bracket: no big deal, replace by a XT when it's worn (to my experience, the XT is more durable in rough conditions, the bike given in picture came with "Deore" (BB-MT500), and has been replaced by a XT (BB-MT800), in terms of distance, the XT has already been used more than the Deore).
Chains: I remember there was a comparative for road that said that Ultregra (=road XT) were the more durable Shimano, but when off-roading, you need to clean the chain more often if you want it to last, so some prefer to take the "Deore" and replace them more often.
Cassette: the main difference is weight.
Pedals: that's a personal choice, depending on whether you want flat or clipless (or hybrid). On sport bikes, I prefer clips, so I replace directly. On utility bikes, I prefer flat, so but replace when it's worn.

On the bang for the buck, I would rather say that Deore is the best in general terms, and XT is if you want the additional features (that are mostly refinements), SLX should be "better built" than Deore, but without the XT features. If you want to optimise, you can mix and match. For example, take an XT shifter and a Deore derailleur (which is the opposite of what manufacturers do, they prefer to focus on the "visible" components). That being said, there are also personal tastes: XT shifters are more "firm" than Deore/SLX ones (but are more "reactive"), so some prefer to have a Deore/SLX shifter because they are more smooth.