MIA-TPA, perhaps not. That's also a workable air market, to be fair. However, the TPA-FLL/WPB markets are probably a different story (and the situation is probably more favorable to Cocoa or Stuart/Fort Pierce as well). Also, time of day does matter here - South Florida's traffic is quite unpleasant to drive in, so I think a lot of folks will take a pass if they can help it.That's the problem with most long distance trains. Not enough demand to justify more than one train per day for most routes, so, by definition, many places will be served at horrible times. I wish the Silver Star would run 6 hours ahead of the Meteor to get some variety in the overnight schedules.
I really don't see Brightline to carry many people from Miami to Tampa. It'll mostly be MIA-MCO, and MCO-TPA or points between. It just won't make sense to take a 5-6 hours train journey when you can drive that in less than 4 1/2 hours.
As to the Star/Meteor, pulling the Star earlier by another three hours or so SB would get Columbia out of the graveyard but you'd have Jacksonville land there. Pushing NB later would do the same thing. However, in both cases the train would get a bit "bunched up" with the Palmetto/Carolinian north of Rocky Mount (the SB Star/Carolinian would only be 30-40 minutes apart, though I don't think it's unreasonable to sort-of swap their timetables and have NC run one of the Piedmonts to connect with the Star - as of now, the Star has no such connection; you could probably also just use an existing train for the connection from NC heading north).
[What's really needed on the Florida service is more trains, but that's a tired refrain that will probably not stop being true in the foreseeable future.]