If your connection of 45 minutes is the one given to you by the Res system, ie it is a "legal" connection, then I would not worry too much.

AA will allow even shorter connections for people arriving off American Eagle from the Bahamas and connecting internationally, if need be they will lay on a special bus on the apron and take you directly to your aircraft.

AA have a good record of prioritising connecting pax at MIA, reminds me of the underlying core reasons I fly AA :
- On time record
- No lost baggage
- Ease of connections

At the end of the day, the elite privileges and MRTC/B are gravy, getting there on time with minimum hassle is not to be overlooked !