Cool thread. GTD owner Bob Macdonald produced a useful spreadsheet that enabled you to enter all gear ratios, engine power characteristics, i.e. torque/hp/rpm, Wheel diameters, drag coeff (Estimated/calculated or actual if known) even ride height I think and it would then plot speed vs rpm in each gear projecting most advantageous shift points etc. Was a bit theoretical but actually fairly accurate if I remember correctly. Someone on here probably has a copy or I might be able to find one if interested.
We recorded 176mph in Roy Smart's GTD40 Mk1 (168mph in rain), with a Renault Transaxle and it was still pulling, till our injectors maxed out. The following year when injectors were more suited, the motor broke..
With a stout motor and suitable gearing, I reckon 190+ should be attainable - 200 possibly, but you'd need a long straight and a lot of nerve - I think I'd be wary of cross-winds and body lift too - esp nose section.
We are fitting an all alloy 580HP motor/Ford GT 6-Speed into Bjorn Arnils GTD MK1 at the moment and will be looking to see what it will pull later this year.
I would say though that in my experience, it's torque that kills transmissions and operating them for extended periods at high speeds, esp if trans temps are allowed to rise too high.