Hi langy, I have the master cylinder but haven't fitted it yet but I've been driving the car as the every day driver because it's the only car I have registered right now until my vans registered just in case, don't want to be court out without a car, been doing panel and mech work on the van.
I don't think you need to worry much. The VL caliper has one large piston. The reason they went twin piston on the VT was to increase the pad size squeezing the disc to improve braking power. The overall combined piston area between the VL and VT is about the same.
As I've been driving the car with the standard master cylinder the pedal goes halfway through it's travel but the braking power available will bloody well squeeze your brains through your nostrils.... well nearly
The standard master cylinder piston is 20.6 the 75 22.2 It doesn't sound much but the 75 one is much physically bigger. I'm fairly certain it will do the job well and I am planning to post about it here when done.
For reference it's a 75 2.0L master cylinder, part number FBM4006 That can be cross referenced with a lot of other numbers.
The only difference is one pipe needs to be reshaped "by hand" from the green arrow to the blue, to easy!! I hope you fully go through with it you wont regret it.