Focusing on your desire to outrun small storms at "20 knots or more" while crossing the gulf. I can't remember right off the bat how many times I have crossed the 165-mile stretch of the Gulf of Mexico between Carrabelle/Dog Island and Tarpon Springs/Clearwater, but if that is the biggest reason you want to be able to power up to 20 knots or more, let me allay your worries. There is enough accurate weather forecasting available that you should never have to run that risk. I have not even seen a thunderstorm out there because we wait for the famous "weather window." High RPM turning turbo-powered diesels (like my current engine) are a horse of a very different color compared to the two naturally aspirated 120-HP Ford-Lehmans I had in my trawler. They are now 51 years old and going strong under new management.
Getting a comfortable enough boat in the 36-40 foot range to go that fast is going to require a much more diesel power (and thus likely restricted maintenance access) than is required for much of the loop unless you don't care to see what is passing by during your travels, which from the tone of your post seems untrue.
A few questions you might want to answer to yourself in your quest are:
Do you want to be able to operate the boat from enclosed comfort?
Do you want/need a flying bridge?
Single or twin? Probably single since you want ST and BT. Ah, more maintenance access.
Galley up or down? My wife always enjoyed being next to my pilot chair as she puttered about fixing meals in our galley-up Grand Banks 42 so she could see what was going on and help lookout.
Electronic:
Radar? We have encountered lots of fog on the rivers.
AIS transponder? I would not leave home without it. The river locks now have it and seeing you coming can help them help you get through faster. Helps you see the towboats around corners.
Plotter: Of course.
Auto pilot: I would not leave home without.
Sounder(s): At least two. I have been on the rivers when the only sounder on the boat I was moving went out - made me feel naked. My own boat had three transducers.
Night vision system: If you insist.
Beds: Sleep in the bow and put up with water chuckling under your head and more vertical movement if it blows or aft where things are quieter?