I'd be very wary as with anything which quickens the shift will place more strain on the synchros, which are already a known weakpoint on these. In fact you should be very well aware of this sort of thing having an MA box in your Rallye!! Satchshift's wern't out when I had mine, I just had the TAS item which was basically crap.
As long as the pivot points are all properly moved and so you're not looking at masses of increased leverage and force, then they might be ok....but for the shocking amount of money they cost I'd give it a miss IMO. I mean £800 on a shifter which if you reduce the throw will probably **** your box..... actually you gotta be nuts to consider it!