Tampa-Miami is going to be interesting if it comes to pass. Brightline is likely to land in the ballpark of 4:30, maybe a bit more (I'm assuming a hair over 3:00 MIA-MCO and another 75-90 minutes MCO-TPA - I don't think they'll match the Florida HSR estimate of 1:04, and we don't have MIA-MCO timetable for them yet).
On Amtrak, TPA-MIA is timetabled at 5:26 either way as of right now, but it's possible that on a "local" train making the same stops (but not dealing with LD ridership), you might see 10-15 minutes trimmed out (depending on the LD padding situation vs the train presumably not being in "discharge only" mode, as well as any "sticking points" or slow spots that get fixed because of this project). So the time difference is looking like it's on par with some of the longer pairs on the Acela vs Regional front (and the pricing might be similar, too...)...
...and this is presuming the current timecards. If we go by the Amtrak route proposals, MIA-ORL and MIA-TPA both come in at 4:30-4:40. So that's...maybe a 20-30 minute difference for TPA-MIA, presuming neighboring stations. ORL-MIA vs MCO-MIA is trickier to pin down - the paper difference is probably about 90 minutes but accounting for getting to Brightline it's probably closer to 50-60 minutes. That being said, the catchment areas do differ (and I'll argue that if the folks planning this are smart, "Orlando-Miami" will run to WPK, SFA, or DLD as a terminus instead - there's no reason to pass on folks wanting to go from the north side of Orlando to South Florida/force a connection on them when you own/control the tracks, especially since getting across town is a pain at the wrong time of day).
[What can I say...if we want this sort of frequency back, dueling service situations were inevitable...]