For what it's worth, we shouldn't toss out 7, 7.3, 7.6, 8, 8.3, and 8.6
That was the sweet spot in terms of experienced players and that some of us were slightly better than others of us (myself, I'm probably really only a 6 on my lonesome, but with the friend that got me into AI War in the first place, we'd play on 7.6 most of the time). And it had to do with the exponential nature of some of the effects, so jumping from 7 to 8 was really big--as big as the jump from 4 to 7.
So, whatever the names, the "values" I'd keep would probably be:
2, 5, 6, 7, 7.5, 8, 8.5, 9, 9.5, 10
Which...interestingly...makes for 10 difficulty levels.