From what I have heard at FECRS Meetings, there is no significant change in seed limits for Intermodals/freight, which according them were already running at optimal speed on their already excellent track
. The higher speed limits are entirely a passenger thing.
What has greater impact is the double tracking which eliminates the need for arranging crossings which involves holding on train in a siding. The flip side is, some freights may have to hold for a Brightline to overtake them.
We could more firmly ground this discussion in reality by taking a look at this FDOT document:
New River Crossing Feasibility Technical Memorandum (FDOT) (PDF)