There’s context needed here.
Riding to power in an actual XC race (under 90mins) on a ‘legit’ course is essentially impossible. Not, if you’re trying to win. There’s just too much happening. Power is insanely staccato in a MTB race. If it’s not, the course is likely not ‘legit’.
I’ve done a fair amount of XC racing and a lot of solo XC race simulations. Without doubt, the most important tool for pacing is RPE and intimate course knowledge. Having a precise internal perception of your ability to tolerate varied over and well over threshold efforts during your race, is a huge factor in success.
On a personal level, my race simulations taught me precise pacing, without any metrics. In fact, I’d say without these skills a rider could never reach the absolute top echelons of the sport, in any categories. Track riders are another perfect example.
There’s no question that a significant proportion of current amateur road cyclists are far too reliant on metrics. I’d go as far as saying that if I was coaching young riders, even if their goals were road. I’d have them do 60-90min MTB XC race simulations with no live data. These individual simulations are possibly the hardest efforts to pace, on any bike.
This high level varied pacing skill would give them an advantage over regular road only cyclists. Additionally this XC MTB training, also gives any rider, better cadence adaptability and greater seated power. Not to mention significant bike handling benefits.
I think it’s been well demonstrated now, that some cross disciple cycling training, particularly when young, gives substantial benefits later in the riders career.
I do believe that you should record the data, both power and HR. As, analysis is very useful, post effort. This data helps inform your next effort.