I was thinking that maybe there should be some sort of reward for playing on Ironman mode.
I remember when I played Fallout: Tactics in Ironman mode, your soldiers would get an xp boost and level up more quickly.
Something similar could be applied to the Ironman mode in Xenonauts. Those brave enough to play on that difficulty could expect their soldier's stats to increase more quickly. The degree to which they do would have to be balanced. I doubt it'd be fair to give them double the stat increase, but maybe a 25% increase over non-Ironman would be fair.
I guess how easy this is to do depends on how the counters for stat increases are implemented in Xenonauts. If counters are reset between missions and don't carry over from mission to mission, then a 25% increase wouldn't make a difference unless a soldier's stat increased 3 or more points (assuming you're rounding up) in a single mission, and more complicated mechanics would be needed to implement this. But if the counters do carry over from mission to mission, then adding a simple 1.25 multiplier for each counter in Ironman games would be pretty easy to do in my opinion.