I rode and raced XC and endurance mtb events with Shimano M596 (previous generation Deore before M615) hydraulic brakes all last year, they're great brakes for XC, trail riding, aggressive XC, etc. If you were to buy a nice downhill-specific mountain bike equipped with Shimano hydraulics, it would likely be equipped with Shimano Saint brakes, Shimano's brakes for more agressive gravity-oriented riding. I'm not a downhill mountain bike racer, only XC, and there's not a lot of that type of mountain biking in my area, so I don't know much about downhill bikes other than reading the specs. Exactly how you're going to ride and where are the most important factors in whether the Deore brakes are a good choice.
I will say that downhill riding and racing used to be done with a lot less brake than the current Shimano M615 Deore hydraulics. YMMV.