I am in the same boat as the OP. No matter what I try, I can't never hit 7 iron longer than 130 yards with interlock grip, but can easily hit the ball well over 130 yards with 7 iron. For some reason I have difficult time holding the grip steadily with interlock grip, especially at the top of the swing. With 10 finger grip, I can hold the grip steadily which seems to impact the down swing & impact.
I've been playing with interlock grip for about a year with no real improvement. I am thinking about switching to 10 finger grip after reading this thread. Glad I found this thread.