you wont loose the upgrades right away, they will slowly decline...
setting up the <25% control of corps for the higher upgrades, and raising taxes to 75%, is one of the most profitable econ strats. and depending on how well you manage corps, can actually make more money than all CEO corps.
as far as high priced shares, IFs will buy shares at 100 P/E, if you look at your monthly net profit, multiply that by 1000, and you'll get the max market value that auto-IFs will buy at. although they will buy a little higher...