Yes, tinctures are most effective when they include some plant "impurities" rather than pure salvinorin A. There are basically two possible reasons for this:
1.) The compounds facilitate the absorption of salvinorin A through the oral mucosa.
2.) The compounds promote the formation of emulsions. Salvinorin A is fairly soluble in ethanol, but the solubility drops rapidly as the concentration of water increases and the concentration of ethanol decreases. As the tincture is diluted by your saliva, salvinorin A is apt to drop of solution (and when it drops out of solution, bioavailability is extremely poor). But if the plant compounds promote the formation of emulsions, they help to maintain salvinorin A in a more bioavailable form (as a component in an emulsion rather than a precipitate).
I suspect the second explanation is correct.
As to preparing a tincture from 5x extract: you'll have better luck with acetone than
i-propanol. Sphere put together some good pdfs on the subject (see, for example,
Salvia Divinorum Salvinorin Extraction and Refinement FAQ)
Another point to bear in mind: Extract-enhanced leaf products rarely contain the concentration of salvinorin A that they ought to. Your 5x might be no more potent than plain leaf. Bottom line: dosing extracts is even more of a crapshoot than dosing plain leaf (which itself can vary pretty widely in salvinorin A content).