I have been wondering... how bent tot he leathers have to be to comprimise response of the reed?

I have some that don't lay flat, but they're not extremely seperated. I keep thinking I'm putting more effort in playing my C accordion than my D. But, I also have to factor in that a C has bigger reeds and a bit more air consumption?

Anytime the leathers are not laying flat there is a small delay in responce. Usually not enough to effect anything but when the gap is over 1/16" you may hear a little flapping noise when that leather stop closes as you change direction of the bellows.

The harder you push the bellows the more the leathers will curl.

FWIW ..

I commonly see that tiny "helper" springs are
used in piano accordion leathers.

The springs are simply a short length of fine spring wire
that rests upon the leather, holding it gently in
closed when at rest.

I don't know if these are ever used on Cajun boxes?

--Big

Yes,
they are commonly used on the bigger reeds.