This is just my opinion but I think the 1st four corps troopers could use a little attention (Rebel, Fleet, Storm, and Snow). But I DO NOT think the unit should be discounted. Instead I think the fifth trooper should be half the price. This way the unit is stronger with an additional health and attack die at a discount. If you lower the price of units I am afraid we will just keep seeing more activation spam and less people finishing 6 rounds of play.
But I also think an activation tax could help with too many activation. A player would pay 3 points for every activation over the required 4 (1 commander, 3 corps). A player with 9 activation would have (9-4)x 3=15 points in activation tax. A player with 13 activation would have (13-4)x 3= 27 points in activation tax.
Again just an idea.