Not sure about your specific gravity theory, but a shot will taste different down the demitasse. I frequently find the last sip to be the sweetest, and yes, quite punchy.
Another variable to experiment with. Pull a double, but with a demitasse under each PF spout. Drink one without a stir, drink one with a stir. See which you like best! It may change depending on whats in the grinder ;)
Had an amazing Limu a few weeks ago that was awesome with a quick stir.