I don't know if my mast leaks, but it will turtle if i don't stand on the hulls quickly. I also have a righting bar. If it does turtle I can just grab the righting lines and lean out over the hull, or set the righting bar to lean out, and the boat will SLOWLY come up so the mast is nearly horizontal. From then it's just a matter of having enough weight to right it, but if there's water in the mast, that extra weight at the end will make it tough.

I think plugging the top of the mast will definitely help in terms of leakage. Just think, when you're turtled, water can only come in the top, which is the mast base. For water to get in, it has to get past the air which is escaping. Now if you had a hole in the bottom (mast head), any air escaping from the base will be quickly replaced by water coming in the head. Plus you have the added pressure of water at the head which is 20+ feet below, helping to force the air out.

I'm surprised you weren't able to get it up from a turtle. When the whole rig is under water, the water in the mast isn't 'extra' weight, you just lack the bouyancy of a mast which is hollow.