I don't like the 5 upgrades limit.  I was hoping that you would eventually be allowed to have more than 5 upgrades at a time.

Possible alternatives:

1. The more stars you get in missions the more upgrades you can have (I really like this because your power can snowball.  If this makes things too easy just let the player up the difficulty)

2. The later the mission (example: mission 1 can have 5 upgrades and mission 2 can have 6 upgrades) in the sequence the more upgrades you can have.  This helps solve the problem of you getting more and more turret options in later missions, which means the 5 upgrades limit is spread out over more and more turrets, which is not fun.