MD80Rookie, intriguingly enough that level flight demonstration is exactly the technique my colleague has offered to use as a demonstration of HIS point. He claims that the N1 will not change in level flight at 230 with the slats extended or clean. Obviously this does not make a sensible argument as, like you stated, either the power will eventually stabilize at a slightly higher value to maintain the same speed or, (with the autothrottle off) the speed will eventually bleed off a bit. Either the result of..... DRAG.
After he demonstrates his bit, I plan to demonstrate this to him and beat him using his own trick. I await the demonstration enthusiastically, and will share the results. I REALLY want my free drink.