Yes - Ace did tune down a half step (all of KISS did) and used 9's, although he also had a custom thin/thick set by Gibson for a while too.
The 1/2 step down will work even better on the longer Strat scale than the shorter Gibson scale.
As you tune down, you want thicker strings to maintain tension. In this case, you get a tension boost from the length of the neck.
I have 9's on all of my shredsticks in Eb - Longer scale Jacksons. They work great.