I run my 20 in. MX RSL with a 100mm Fox, and have used both 80mm and 100mm forks on a Mooto X. I'd guess the geo changes from the 100mm fork are no more than one degree slacker (perhaps even less) to the head and seat angles when compared to an 80mm fork. Keep in mind that A-C measurements between different fork makers are not always the same, i.e., a RockShox 80mm fork may not have the exact A-C measurement as a Fox 80mm fork. Also, some companies measure bike geometry with a bit of sag, which doesn't always equate to how any particular rider sets up their own fork/bike.
All that said, they're just numbers, and numbers with small differences at that. The choice between 80mm and 100mm for a Mooto X or Mooto X RSL essentially comes down to personal preference. The frames can handle either and perform well, although the handling characteristics will perhaps vary a bit depending on the A-C length. A tiny change in bar height, stem length, or saddle position can pretty much balance out any weight distribution changes brought on by changing the fork. The bikes rip either way!