I had a VL Turbo, as did some of my friends. One of them ran an 11.97 on a standard engine with bolt ons and standard computer. I dont think a aftermarket computer is totally necessary with these cars unless u want to go super fast. Mine put out around the 250rwkw mark and would have been capable of more if i installed an aftermarket computer. Now that they are getting on in age, a rebuild is probably wise as the engines are getting a little tired. I gave mine a freshen up with a complete engine rebuild (top&bottom end) and used ACL pistons and it ran 18psi all day everyday and was a daily driver.
The manual MX7 gearboxes are extremely tough provided they are maintained properly, i never had any problems with mine and nor did my friends who ran constantly at heathcote/calder park with a lot of horsepower running through them. Automatics... if left standard they wouldnt cope with a lot of power... my friends standard vl turbo running 12psi with a supra cooler, the auto box held up for about 6 months with a bit of abuse... on the other hand you could go a trimatic or a jatco 4 speed, in my experience the trimatics held up a lot better. It depends whether you want fast times or not (obviously automatics will provide you with better times). Some people replaced the manual MX7 gearboxes with supra gearboxes, i didnt think too much of this as they kept breaking these boxes, i'd look into the ability to fit an RB25DET gearbox.
Concerning turbos, once again i think this will depend on whether you go manual or automatic... if you go automatic and have a decent stall converter in it, a t04 will be ok... I had a T3/T4 hybrid turbo on it and it was great (built by ATP) as lag is a big concern for me. I have been in near standard vl turbos that have T04s in them (a bt1 with a cooler and manual) and it only made decent boost at 4000rpm, and then having to shift at 6000rpm, so you have 2000rpm of decent power and when you change gears you were out of boost, so it was an absolute pig.
Make sure you have the correct turbo combination on your car, bigger is not necessarily always better, useable power is the key.
Common modifications on a VL Turbo to get yourself into the low 12s high 11s mark:
Fuel Pump
t3/t4 hybrid turbo
3" exhaust from turbo back
intercooler
decent clutch (heavy duty or button)
you can keep the standard 3:4.5 gear ratios in the diff, but 3.9s or 4.11 will definitely help.
cold air intake (the standard airbox works well, just cut a hole in the bottom of it and feed the cold air intake into this)
extra injector or bigger injectors
Cam
Catch can
Thats all i can think of for now... these arent massive modifications, and all the times that my friends ran were on slicks and using aviation fuel and soft springs in the rear end. a T04 depending on the type and the condition its in can run a fair amount of boost, it just depends on your engine - what condition is your engine in and can it handle large amounts of boost.
Hope this helps some what.... =)