To interupt the argument for a second.

I made the changes you suggested LRT, unfortunately I also got carried away and broke the cardinal rule of logic and changed some other stuff on the spark tables and speedo calibrations, which ran like crap and couldn't test the transmission state in anything other than idle basically.

When I got the car up to around 40km/h (maybe 15mph) and let off the throttle the transmissions felt like it was locking up and releasing over and over again for 0.5 seconds in succession until I put my foot back on the throttle.

Obviously the behaviour is bad, but is this a good sign that the transmission is ok and that I just need the tuner to calibrate / swap out the transmission segment, or is it a sign the transmission is faulty?

Also to throw it out there, is it possible the cause is that the transmission selector cable is not set properly and the trans could be between gears/engaging both?