I went with only one grate, 7-8" height. I have never tried a big turkey before, but that is probably about the only thing that might not fit. I think you are fine. If you find that your bottom grate cooks too hot, add a deflector plate or water pan before it. Benefit of a UDS, it is never a complete project, you can always do more.
XL Big Green Egg, Weber Performer Platinum, WSM 22.5"