Not sure what you mean by overhead, i'm kinda new to this stuff. Does this just mean there's more processes to run kismet code?
As a game artist i know what it's like when things aren't optimized. Put something in with to many polys and the engine really chugs. I've been doing some really crazy kismet stuff and so far i haven't seen the game engine bat an eye. So as someone who spends hours optimizing things to make a game run faster, kismet really doesn't seem that bad, even when it gets really gnarly.
When you think about it, how much processing power do game engines these days spend on the actual gameplay. I don't mean the physics and all that, i mean the elements of health, ammo, damage computations, etc. It's really not that much. A little overhead in this area i feel isn't going to hurt anyone.
And again, i'm no coder, i'm just having a real blast goofing around with kismet in a sandbox environment where it's easy to adjust gameplay very quickly without having to type it out.