At the risk of sounding like a spokesperson for Ascent, which I'm not, I believe the intent is to use the Texan to bridge the gap between the Prefect & the T2 in performance terms with advanced simulation to ease the transition, not to replace the T1. The Prefect cse would cover more than the old Tutor/ Bulldog/ Chipmunk syllabi so they are not really equivalent, the Texan I guess doing higher speed LL Nav & higher energy manoeuvres.
Personally I think the RAF & the other services could've done it better themselves for the £3 billion the PFI will cost but the projected 'saving' of £1billion was obviously too good to resist coupled with the post-RAF career opportunities for a few VSOs.