It will depend on yeast health, and type of yeast.
FermCap does not seem to do anything to Belgian yeasts. If you have a healthy pitch with a Belgian yeast, it will blow-off with a 0.5 gallon head space.
I use FermCap and have slowly been reducing my headspace. I've gotten to around 0.5 gallons (in a 7 gallon fermenter) and have started getting blow-off in every batch, when I didn't before. It's not much, but enough to fill the airlock, and if I don't get to it soon enough will plug the airlock and blow it off.
I'm back to using a tube on every batch again. I don't lose any beer (except for Belgians) as the foam does not get high enough in the tube to go out the other end. I use a swamp cooler to keep temperatures in check.