Not sure if this is intentional or a bug. When you stuff any improved/superior/supreme herbs into a healing kit, and use the said healing kit, the effects are that of normal medical herbs. I have tested this mixing herb kits with different kinds of herbs to see if they stack, and also stuffing kits with only improved herbs, to no avail.
The reason I'm wondering if this is a bug is that a DM told me he thought this was supposed to work, and I feel that it should. If the case is that this isn't intentional, I would love it if there was a way to get some kind of healing kit for improved herbs.
I've always assumed this was intentional. If you want to keep improved/supreme herbs separate then you can. I do like the idea of "improved" or "supreme" healing kits available for healers though. It would make investing in heal and crafting herbals far more useful. Their weight adds up quick w/o a kit.