Switching elements is a thing, but this kiln already was designed and calculated using uniform graded elements. Besides, the voltage drop produces unwanted heating wherever it is occurring (wire, connections, sub panel, breaker ….) which then becomes a fire hazard. Find and fix the issue first and foremost. Applying load changing techniques will actually increase the heating in the defective areas (wire, breaker, sub panel …) for which they are likely not designed or rated. So really a last resort that requires verification for entire system safety appropriately and then this kiln cannot suddenly be used at its standard rated voltage so marking appropriately. I would suggest It’s really something that can only be done by those appropriately skilled in doing so as it can increase the risk of fire in the whole system without appropriate precaution and verification.
Like Bill said, you can't just put the T/B elements in the centers. Yes, it would give more power, but it wouldn't be balanced anymore, and the middle would run too hot. You could, in theory, replace all the elements to compensate for the low voltage, but they would all have to be increased appropriately to maintain that balance, and the whole system may have to be modified to deal with the increased amperage draw. Fix the voltage issue and everything else will work as it should.
