This interests me also.
I have not done this but have also begun to look into using a stepbore master in a 4 wheel disc manual application.
The step bore section of the master cylinder only works until about 100 psi system pressure is reached, as it is only designed to bring the pads up to the rotors, then the valving in the master lets the smaller bore piston take over to actually build the pressure. The pedal effort to reach 100 psi line pressure will be minimal with any pedal.
Think of the stepbore master as you would a 2 stage compressor, where you can get the necessary volume AND higher output pressure with less HP.
So I would imagine it is not a big deal as far as pedal effort is concerned, as long as you can construct the pedal so you can have a 6 or 7:1 ratio AND still have the pedal configured so that you can have the pushrod actuate the pistons through their entire available stroke.
This is not as hard to do with a swinging pedal as it is an underfloor pedal, as you can always bring the pedal up higher to get your pedal proportions right.
From my experience, if the secondary bore size is 1" or so, a 6:1 ratio will be adequate to apply enough pressure to the metric calipers, you will have a stiff pedal, and a 7:1 might be better if you are going racing and need to build some higher line pressures.
Do you know the bore size and available stroke of the stepbore master you are looking at? A couple of the step bore masters I had looked at had 1 1/4" primary bore, and 7/8 secondary bores but never investigated the stroke length.
None of the aftermarket companies I talked to had anything to tell me about stepbore masters, but I only asked a couple of them, and it's hard to get to a real brake engineer at those companies.