Ata
Your missing the point. The 280 is an improved tiktrotor design with increased agility and fixed engines with all of associated benefits (minimizing surface heating, improved firing arcs, simpler engines, etc...).. Itt was never meant to be significantly faster than the V-22, but was meant to have significantly better hot/high performance. The time to max speed is only relevant with respect to showing the maturity of the technology and technical prowess of the developing organization.
The 280 program has done what it needed to do. It has already demonstrated superior performance and lower technical risk as compared to the SB-1.