I'm kind of a bit hoarder to be honest. That's the thing about us primary barrel racers, we've got a bit for everything!
The combos both have gag as well but you are right that the elevator has a lot of it. I like gags because they don't immediately go to full action. They'll ease into it. However, I wouldn't put them low hanging in the mouth like you said, otherwise you'll have to pull a LOT to get the full action if you need it. I use them on naturally lifty horses because they will put on more poll pressure that I often see putting horses on the forehand if they don't have that natural shoulder lift and drive from behind. If I have a horse who doesn't lift quite as well I'll do a tiny bit of gag (Like the combos) or a fixed shank.
A good thing to do with horses who do these types of events (Like the jumping and the barrel racing) is keep that "Go-time" bit and the "Slow-time" bit seperate...My mare does both the reining and the barrel racing, so she knows what we're doing depending on the bit. I'm sure your horse would pick up on that easily after he learns you will only jump in that combo bit.