I put big brakes (r34 4 pistons) on the silvia on the weekend. Left the singl piston on the rear. Started bleeding brakes and realised that I forgot to swap the master out for the new bm44. I couldn't be bothered and was interested to see how bigger calipers would affect pedal feel, essentially the same as reducing the master cylinder for the same caliper. What I found was that it feels a lot more like the corolla. I feel with the corolla, that braking force is proportional to pedal force but it travels a lot. When I hopped in the silvia(stock brakes), I felt that the brake force was more proportional to pedal travel than force. Putting big brakes on it made it feel like corolla again. It's a bit weary now that I've gotten used to how the silvia felt but I think I prefer how it is now for feel. I think I can control the car on the braking limit better.
Going up in brake master will give you the opposite of what I described.
You're on the right track. Pedal effort should be more a result of the ratio of the master cylinder cross sectional area and the caliper cross sectional area. For example, say that you increase the total cross sectional area of your brakes by 10% and increase the brake master cross sectional area by 10% as well, the pedal should feel exactly the same as standard. I'm not too sure about piston size comparisons between Corona brakes and Celica fronts/Tarago rears, but there's a fair chance that if the pistons are larger than the standard Corona ones, increasing the master cylinder size should improve pedal feel.
With the booster/master cylinder pushrod clearance, there should be a minimal gap, but it's probably advisable to make sure that there actually is a bit of a gap so there is no chance of the brakes lightly engaging when driving around.
Hope that all makes sense.