"A bead of 5200 or silicone around the mast should finish what the Spartite started."
I don't think you want to use 5200 anywhere you anticipate wanting to remove it some day. The Spartite will age and eventually need replacement, I'd stick with the silicone. Definitely don't use 5200 on the mast collar edge of the Spartite, or the mast won't come out of the boat...
"Or do I need to take other measure for sealing the rubber boot?" depends on what is neceassry so your boot doesn't leak...More silicone, remove, clean and refit the boot cover, new boot cover, or converting to Spartite, whatever "no drip" requires. I am now at the "coverting to Spartite" stage.