Once I did a try with T probes up and down, w/water in water pan. During all the session the lower grate was 25 F less.this is my experience.
15- 25* is a big difference, but despite that difference in temp, I find that things on the bottom rack never cook that much slower than the top rack, at least not nearly as slow as what you'd think. I find the difference in terms of how it affects the cook is the most when cooking with an empty foiled pan.
I measured my 14.5 the other day with a maverick and two grill probes, the top was 50 f hotter, this was with a full water pan