I race XC Cat 2 and MTB 100s. My setup is a flipped stem to give it a negative rise (-6 degrees) and I have one spacer above and one below on my Scott Spark RC 900 WC. I also tried the stem slammed (no spacers below) for about a month but didn’t like the feel. A slammed stem is what most pro XC racer do, but I wouldn’t try to mimic theirs or any other’s setup.
Regarding XC racing, wind drag and becoming more aero should be about the last thing you should worry about concerning most (if not all) MTB racing events. Becoming more aero would be at the utmost end of marginal gains once you have optimized everything else. Slammed positions on XC bikes are generally about handling steep and technical uphills, not about getting more aero. Conversely, that slammed position makes it more difficult on technical descents. So the trade off becomes the benefit of keeping more weight “on” the front wheel going up, but being able to handle that somewhat compromised position going down. However, since XC races are generally won on the accents that compromise is worth it if you have good handling skills.
The priority of your cockpit setup for XC racing should be handing, as you’ll lose far more time being inefficient in that aspect than you’d ever gain by having World Class CDA. Furthermore, the nature of XC racing and the actual speeds for those events, further minimize aerodynamics benefits and are far less of a factor (when comparing to road). I just looked at my last five XC races from 2018 and my average speeds for those races (where I placed between 2nd - 4th) were 13 mph, 15 mph, 12.2 mph, 14.2 mph and 13.7 mph, for an average of 13.6 mph.
Lastly, for 24 hour races and MTB 100s (8+ hours) comfort will be especially more important than typical XC races that last 1:15-1:45. Being on the bike for a relative extreme long duration, and being beat up my rocks, roots and drops, is far closer to survival than being optimized for aerodynamic drag. Anything that can be done to ease that survival (comfort) will have far greater impact on your outcome than any perceived watts gained improving CDA.