The others here that are experts will give you more info but you will have a difference in temps between the upper and lower racks simply because heat rises. The difference in temps between the left and right sides will vary slightly as well. Although the left side (opposite the firebox) gets hit with the smoke first, the heat still starts at the FB and moves left. Depending on how thick your baffle plate is it can be a factor as well. Thinner metal will heat up faster but also won't hold heat as well. The thicker the better for even heat distribution. One thing you can do to adjust the temps left to right is to either raise or lower the firebox end. If i am correct, raising the firebox will essentially force the heat to stay on the FB end longer and build up. Raising the other end will have the opposite affect by allowing the heat to flow out faster evening out the temps. My RF is only a 60gal but by leveling it I am able to get the left and right sides exactly the same.. My top rack is about 40 degrees F higher than the lower. That works fine for me because I can do ribs/chicken up top where i can use that extra heat.. Hope this helps.
The others will correct me if I am wrong but I would think that the location of your stack can have an affect on the heat as well. Lower in the chamber to me would force a bit more heat to remain in the top of the chamber. My stack is about 1 inch above my bottom grate forcing the smoke to be pulled down before it exits the stack.