Hi Rob, I'm one of the happy new owners of the 21" kk. Like all the other owners of all the various sizes, I absolutely love my new cooker. It is every bit as awesome as I expected it to be after 13 years of dreaming of owning one.
As regards your questions about high temp cooks like steaks and pizza, I'm sorry I can't testify yet, because I've only had mine about a week, and I just now spent the last couple days doing the preparation burn-in and grouting so I could do those. I've got a couple waygu filet mignon in the fridge that will be my inaugural high temp cook, and I will be doing a pizza soon... As for concerns over the size of the stone, you might consider that there is no law that you have to use that stone. I have a 16 inch round stone that I used on my previous inferior cooker, and I am certain that if I wished to do so, I could raise it above the main grate using any number of methods, i.e adjustable rig, and there is more than enough room to allow that. I would suggest no reason for concern over whether or not there is enough room to cook a proper pie. I chose the 21" because the price point passed my wife's approval, and because I felt the size was perfect for the two of us. It's also plenty big for me to cook four to six or even eight slabs of ribs when I have my buddies over for our twice monthly card game. I should have made those guys contribute to the cost of the cooker because man, have they got it made now... So, while I fully agree with others that if budget is no issue, then it's worth considering going as large as you can, I would also say that unless you need to cook for more than 10-12 people, the 21" is sufficient for all your needs. I really was impressed how roomy it was when I got it - I was actually a little surprised when I first opened the dome. Dennis is an absolute pleasure to deal with - he is friendly, informative, a great conversationalist, and in every conceivable way probably the best man you will ever have professional dealings with. He and the Komodo Kamado have my full and unreserved endorsement.