I have sailed all sizes: F2 Missile XS, Tabout Manta 49 & Manta 54.
I mostly sail at Strand Horst NL.
Which is shallow, and inland. So gusty winds. But VERY flat water.
A full-on speedboard can be sailed earlier than a high-wind slalom board with the same volume.
No idea why, but others around me confirm that too.
The Missile XS (44 cm) is great, when conditions are there.
The problem is, that is never the case, unless you have a job with lots of freedom and live right next to a speed spot.
So you do not get sufficient TOW (time on the water) to get it dialed in, and get confidence togo full-throttle.
I did get 83.5 kmh on that one (my PR) but that was before my son was born.
And from that moment on, just could not prioritize sufficient time. Impractical for me. I sold the MS.
The Manta 49 and 54 are ideal combo.
On the 54 I can run a 7.1 sail, and still be efficient.
On the 49, the 6.3 sail is spot-on. But the 7.1 feels too heavy, I think 6.6 is the max for that one.
The 49 and 54 sail differently. The 49 is a speedboard, the 54 a highwind slalomboard.
The starting speed of a speedrun is higher on the 54. But the acceleration through, is less.
So the 10s speed of the 49 is better. But you need a suficiently long gust. Or better even, a double gust.
I would recommend to start with a 53-54 board. And get a proper really good fin.
Once you feel you exhausted that from a performance POV, then it's time for a 49-50 board.
When you can afford it, buy the Tribal in 53 size.
I have witnessed that one carries a 7.0 sail without problem (mid wind) and is superfast. If I had the money...
www.tribalwindsurfing.com/products/rapid-speed