I did read your blog jfm. It was very informative. I've haven't come across anything about maintaining a low and slow temp on the xl. I can see how that may be an issue due to the thermal mass of the xl. I'm headed to Las Cruces, NM today to take a looks at them. I've never had a dedicated charcoal grill, so I'm sure it will be a learning experience.
After I made my post here this morning, I went to look in one of the Egghead forums just to make sure I wasn't talking crazy talk. There were posts there about people having issues going low and slow, just as you suggested. Also as you say it is the large thermal mass of the XL that seems to be the issue here. Also the lower draft door is bigger on an XL and lets in more air, so you need to make sure you shut it down soon enough too. The trick is not to let it get over the temperature you're shooting for and you won't have any problems.
Like anything else you're going to have to get used to your new toy. My biggest problem starting out was not knowing the general area where the draft door and the upper damper should be to achieve a certain temperature setting. The first couple times you use your your Egg you are going to have to guess, how much the dampers need to be open for a given setting. I usually start shutting things down 50° before I reach my target temperature. I keep an eye on the amount the needle is rising if it's still climbing fast I'll close down more and more. Always better to undershoot a little than to overshoot. Also try to avoid shutting down the dampers all the way at low and slow. You don't have a lot of coals going at that point and if you overshoot and have to shut down completely, you may put your coals out completely. I've also found from experience, and reading up on various forms that the best way to go is to tailor the amount of paraffin starters you used to the temperature that you're shooting for. So if I'm shooting for 225 I light one paraffin starter. If I'm doing pizzas at 600 or 700 degrees I light four paraffin starters I spread out around the perimeter of the coals. Or if I'm going to be doing something where I need the whole grate area, then I light more paraffin starters. You just need to be careful to shut down soon enough so you don't overshoot.
One last thing: I have a Tips area on my site and I did write a tip about taking photos of your variou grate or damper settings on your new grill to help you learn where you need to be for various temperature ranges. I'll put the link below, and it will give you a ballpark idea where you might need to be on your XL. Just remember your grill settings may be somewhat different than mine. But for particularly for low and slow you'll see I had the draft door & daisy wheel damper open only a little bit and not midway. So at least you'll be starting in the right ballpark when you do your first couple cooks. I have pictures for 225, 350 and 600°. These will get you in the ballpark, but I suggest you start taking your own pictures to make your life easier down the road. That way if a long time goes between sessions at a certain temperature, you'll have pictures that you can look up so you don't have problems. I generally try to make the same mistake once, and find new mistakes to make. Shooting photos of the grate settings help me make the same mistake only once.
Tip Entry with sample grate settings:http://grillinsmokin.net/tips/files/pho ... _grill.php
Good luck I know you love your BGE.