Not too muchAt smoking temps, like 220 to 280, there should be very little differences from left to right. When at 225 on the left gauge, i am at 250 or so on the firebox side upper rack. When at 280 on the left gauge, i am at 295 on the firebox side upper rack.
The coolest spot in mine is bottom left rack which is typically 25 degrees less than the smokers gauge, but go up just a few inches to the upper rack and you are at the gauge temperature. The hottest spot is usually top right when smoking and bottom right when grilling. I bought 6 cheapo oven thermometers and made a chart for every cooking temp in comparison to the left gauge. For example, when all vents are open with food door ajar, and fire door ajar and a good fire, my gauge is at 500 for grilling, I am at 370, 430, and 500 left to right on the bottom and 460, 475, 500 on the top. So the higher the temps, the more the difference.
At 225, I am 210,220, and 240 on the bottom and 235,235,250 across the top. Biggest difference is 40 degrees.
My cast iron bottom grates may hold more heat, but I don't think they have changed its characteristics because they are all the same thickness. They are great for grilling, almost act like a pizza stone for pizza, and they don't rip the tinfoil. If anything, I probably improved the ability for it to recover temperature after opening the door because of the added mass, they are pretty thick and heavy.
Hope this helps,