One is that a sealed air filled mast provides an equal amount of displacement as a foam filled mast, so the bouyancy would be the same.
That's not correct. A lighter-than-water object in the water displaces its weight. It floats. Bouyancy = the object's weight.
If you fully immerse the object, the buoyancy is the total volume of displaced water, less the weight of the object. That's called reserve buoyancy and what keeps the boat from going turtle.
A mast filled with foam weighs more than a mast without foam, therefore the reserve buoyancy is less. The tendency of the boat to turtle will be greater - and the boat will be much harder to right with the extra weight in the mast.
Comptips are not foam filled. They only have foam at the ends - to help seal them off.
Filling a mast with foam is a very bad idea. The added weight will significantly increase righting difficulty and will adversely affect buoyancy in the water. If (when) the mast leaks, water will get trapped in the foam and make things worse. Much worse. (Google the "Sizzler" catamaran and find out what happened to them.)
You're much better off just making sure the mast doesn't leak - and keeping it full of air.