ZestyAren't spy disguise health/uber percentage determined at the time you disguise or did they patch it so it changes now? Otherwise, couldn't you disguise as your med, take a trade, then have your spy activate DR and pick up the medigun to get another uber after the first one already went to 0? Seems more useful than a demo sac.
No. Spy drops an empty medi-gun. The medic uses, switches to the medi-gun dropped by the spy and then you switch back to your own medi-gun before the decay starts to take a good chunk of your remaining uber. The downside of this strat is running a spy.