sonty-
A cruise speed of 450kts+ is probably not realistic with current tiltrotor technology. It would require far too much compromise in the overall aircraft design/performance.
In the US over the past 2-3 decades, NASA and other agencies have published a large number of technical papers covering performance of tiltrotor aircraft concepts, and any of these reports will provide you an analytical approach for estimating the performance of your tiltrotor concept. These papers are publicly available on the DTIC and NTRS websites.