It's like aero modding a Jeep Cherokee. If you're looking for more speed, my advice is to instead invest in a used 10 speed. If you're looking for more aero, look at
recumbents.
Things that will help you a bit on the mtb:
-get yourself properly fit to the bike
-narrower bars that let you reduce frontal area
-build a tubeless wheelset for lower RR. I'm running Surly Rabbit Holes on my single track bike but you might prefer something narrower depending on your tire size.
-a frame bag will help aero at low yaw angles and help you carry more stuff
At least do the bike fit and bars, then mess around with aero, taking hints from the aforementioned link.