Let me start by saying that this feature is welcome in some ways, as there seems to be some tendency in FFAs for players to pile onto the leader, even past the point where he's lost the lead. 

That said, it seems like far too large a drop at this point (as far as I could tell, the effect for a power surge is roughly halved for the second and third hits). That represents a dramatic nerf for the black market, to the point where it doesn't seem particularly effective for evening out the game state anymore. Furthermore, the diminishing returns last even when the tile is transitioned, which doesn't quite make sense to me. I was just messing around in skirmish to test all this, but the idea that a power surge hitting a (freshly building) offworld started at only 20ish seconds was problematic.

I haven't watched the Mohawk stream for this week yet, so I don't know how these things played out in a multiplayer setting, but I'd suggest three changes:

• Slower diminishing returns: something like 90 -> 75 -> 60 instead of 90 -> 45 -> 25
• Diminishing returns wearing off: getting hit with an EMP twenty minutes earlier shouldn't reduce the effectiveness of an EMP in the moment, I think. I don't know what the proper timing is for the resistance to wear off, but I'd imagine it's around the 3- or 5-minute range. If there already is some wearing off in place, I couldn't notice it at all from when I started attacking the AI (when he was level 3) to when I finished the buy, about 6-7 power surges later.
• Diminishing returns disappearing/being reduced on transitions: otherwise this encourages planting valuable buildings in places that were often attacked in the past. In a way, previously goon squaded tiles become weaker places for an offworld, for example (though single-tile attacks wouldn't be affected and are the more likely tool).