Potters & Sculptors - Making Rock from Mud
You have to get to know each kiln's characteristics as they're all somewhat different. I'd ask your kiln manufacturer.
When we have had the odd opportunity to view our Cress E-23 kiln operating above 1,800 F, I've been quite surprised by the size of the gap between the kiln lid and the chamber - perhaps as much as 1/4" in some places. We fire outdoors, so we don't have a down-draft unit, but if we did, I expect the cool air flowing into the kiln from under the lid would far exceed anything we could expect from our open peephole. Can you adjust the flow of your blower lower?
You can also adjust this gap between the lid and the kiln to some degree by sanding down the firebrick in places where it might be too high. But as the bricks continue to shift slightly with each heating and cooling cycle, today's fix might become tomorrow's problem.