I too have the Datson master cylinder. I really couldn't tell the difference in feel between it and the original girling, but it is lighter and doesn't leak my brake fluid all over the driveway, so I changed to it.
I have the same problem with my back brakes not keeping up, even with new slave cylinders and shoes. So I installed the steel brake flex lines in the rear only to try to get the backs to better balance. I've never had any trouble getting the fronts to lock up. I still can't get the rears to, but the car stops well at this point. Not amazingly, but well.
And yes I have cosmic 13 x 6 wheels. One of the bits of evidence that a previous owner raced my car a little is that the wheels, combined witha tiny bit of relieving of the fiberglass in the wheel wells allows my S2 to run 205/60r13 tires at all four corners without rubbing
If you have money, you can always get racing aluminum wheels made to any size/bolt pattern/offset you want. Ahhh, if only I had money. . . .