Yes, the lower rack will run a good bit hotter if there is no heat sink (terra cotta pot, pie pan, iron skillet etc) in place. If there is a heat sink, it will run about the same. This is my experience with the lower rack about 4" below the upper on my first mini. On the new one I've not yet installed a second grate nor have I bothered with a heat sink as I tend to do everything above 275. I do leave the steamer insert in place just about every time I use it.
You'll have fun with your mini!! They're great little cookers and are incredibly versatile. Make sure you make some sort of coal basket as it really ups the efficiency. I made mine out of a piece of mesh gutter guard. Just cut it so it fits under the cooking grate ( I think it's about 3-3.5 inches high) and then made a ring the same circumference as the coal grate. I've also considered using a spring form pan like the kind you use to make cheesecake. Turns out the coal grate is the same size as the bottom of the pan, at least with the pan I have. Before I ever tried it out, however, the idea was vetoed so I just got the mesh gutter guard and made the ring from that.