The hardware of a new board is decided in the very early stages and matches the common equipment in computers at that time, (It takes 2 – 3 years to bring a new board to fruition) which is why Tyros 2 has USB 1.1 as it was the most prevalent at the time.
As to 76 notes, Yamaha research has told them that there is not enough demand for a 76 note Arranger, and this combined with the Unmitigated sales disaster of their last 76 note Arranger, means 76 notes are unlikely to see daylight.
As a rule of Thumb if you want to guess the hardware on a New Hardware based Arranger (From any manufacture) look back 2 -3 years in computer history.

Bill