It must be frustrating for you to have had all this advice and a service at the bike shop and still have a problem, but the basic fact is that XTR V-brakes with boosters is a powerful system and should be giving you all the braking you need if it’s set up properly.
It sounded in the beginning as though your rear brake just wasn’t set up properly. If it has no spring in it or won’t move, it’s either because the spring bars aren’t in the right place or the pivots have locked solid and need fettling. If after fettling they still won’t pivot, they’re probably shot and the brake needs replacing. But if you’ve had the bike serviced since then, I can’t believe any bike shop would let you have the bike back in that condition.
If the brakes are centred and sprung now, but you’re just experiencing weak braking, it could be caused by a number of things (but XTR Vs not being up to the job isn’t one of them) - possible causes:
1. your levers are shot, play in the bushes whatever – you haven’t said what levers they are or how old
2. your cables
3. worn-out pads
4. worn-out rims – old rims can flex so much the brakes have nothing to hold onto
5. brakes set up too far from rims, perhaps because your rims need truing
Basically those brakes should work, it’s just a case of checking each of these things is right.