I have microwaved marijuana and heroin. The reasons for this were; to tighten up a joint by wetting it a bit and microwaving for 3-5 seconds [it works, and if it decreases potency, I don't care, it makes it tighter and its just weed. I've had to microwave heroin because it got wet and I wanted to snort a bit of it, and didn't want to wait for it all to dry out, so I put my normal amount in the microwave and put it on for 3 seconds, wasnt dry, 5 seconds, 10 seconds. I snorted my normal amount and I didn't notice very much descrease in potency, maybe a bit, but it mat have been placebo because I expected it to lose a bit of its potency. This is in response to your post script note about H.
I wouldn't microwave any speed, it won't have any useful effect that I can think of. It just doesn't sound too good, but I don't know for sure.