So doing something that gratifies you-while-high but more severely harms you-while-not-high is likely to be bad for you-on-average.

Well, this gets to a point I made explicit in an earlier draft of this post: viz., how can an outside observer determine the strength of a desire? Meaning, if the heroin addict continues to get high, how can you say those short periods don't outweigh all the rest? (Or to use the example from that earlier draft: how can you say that the burst of satisfaction a suicide feels as he dies doesn't outweigh the entire remaining life he would have had?)

The obvious answer is brain scans or similar, which might easily show a maximum possible intensity of satisfaction, but I have a (probably irrational) distrust of such methods.