By supercells you mean the ones that are extremely likely to spawn tornado, right? I would rather go for not intensity but the amount of time the energy is being gathered.
Here is an additional bit of science: the reason why tornadoes can even form most of the time is the fact air is basically a thermal insulator. It takes so much time to distribute energy due to its high thermal capacity that it just finds a faster way, in this case it's violent mixing.
Long day, for example with two suns that have two separate zeniths about two or three hours apart should be absolutely ideal. It would depose a lot more energy, but not enough to cause convection to be faster then rising and lowering of local air (this causes basically a horizontal vortex that gets rotated into tornado by what is called front updraft and rear downdraft). I do recommend reading, at least cursorily, Forecasting Severe Convective Storms manual for meteorology students.
Bare in mind that I am by no means an expert, just a student who took elective about modern atmosphere physics. However, if you do have questions I can promise you to at the very least ask one of my lecturers if I would not be able to research it myself.