Condenser Capacitiance.

[flatheadmurre]

The colapse of the field should be as rapid as possible to get all energy out to the secondary...a bigger cap slows down this event leading to less output.
Dwell is good getting the coil satured...but if i remember right you break down saturation time in 5 periods...at third you get 78% and 4th 90% so the last bit of dwell isn´t that critical most of the time chasing increased dwell you get a sharp camgeometry and problem with points bouncing at higher speed instead.
For anyone still thinking that the capacitor provides energy in some kind that increase the spark start calculating the energy stored in it compared to what´s in the coil.
The capacitor is a snubber/damper not a power source.