It seems that this question of whether the position of the upper firing pin makes any difference in the reliability of ignition can be answered rather easily with a simple experiment.
First, get your gun and a good supply (say a couple of boxes) of ammo and go to the range or a safe place to shoot. Secondly, be sure that the upper firing pin is not gummed up with carbon or anything and that it will move freely when the muzzle is raised and/or lowered.
Now, all you have to do is load the upper barrel, point the gun straight up into the air and fire the round in the upper barrel. Shoot a box of shells this way and record the number of misfires.
Next, you're going to do the same thing except you are going to point the gun straight DOWN. You may need to go up into a tower or on a cliff of some kind so that you don't shoot so close to your toes.
Now, you load the top barrel, point the muzzle straight down and pull the trigger. Shoot the entire box in this same manner. Record the number of misfires.
After shooting a box of shells each way, you simply compare the number of misfires with the muzzle pointed up versus with the muzzle pointed down. If the test was done correctly, the only variable is the position of the upper firing pin at the moment it was hit by the hammer. In one case, the firing pin would be fully rearward when the hammer makes contact with the firing pin. In the other case, the firing pin would be fully forward (resting on the primer) when the hammer makes contact with the firing pin.
Do the above test and get back to us with your results. I don't think there will be any difference unless you just happen to encounter a few shells which have excessively deep-seated primers. BTW, it might be a good idea to take a look at the primers beforehand to see if any appear to be seated excessively deep. If you do encounter any like this, don't use them in the test.