MBGE questions

Have a MBGE, 2011 vintage. It is a lump miser compared to the LBGE based on conversations with large owners. Can’t speak to a direct comparison as I don’t have a large. 
Gaskets are high temp at my local dealer, lots of choice for after market gaskets and High Que makes a grate - which I like. I use a setter, 5# spatched chicken is no problem, I’ve parted out three (about 3# each). Have done a 12# turkey and 10# butt. 
US ribs run to the large size, they sometimes are too long, so I split them in half to keep them from hanging over the edge of the setter. 
Pizza, I use a13” stone which sits about 1” above the felt line, works just fine. For raised direct I used fire bricks that sat on the fire ring with the grid at the felt line. As most of our cooks were smaller, just the two of us, I use a Grill Dome rack I picked up on clearance for $15. 
I have never had an alignment issue, been true since she was born. 
Less ceramic mass so I think you will find the heat up shorter. 
Good luck with the decision process. 

My wife gave me a Medium a number of years ago, and a Large the next year.  I love them both, consider the Large very much more versatile, but most of the time I'm just cooking for the two of us so I use the Medium very much more than the Large.  And yes, I've done many spatchcock chickens in my Medium (but I think you can fit two in a large, but only one in a Medium).  I don't know what a KJ Classic is, but I have to say I felt a bit limited when the Medium was the largest Egg I had.  I love it, and it's what I use by far the most.  But sometimes I just need a larger grilling area.  I love having both a Large and a Medium, have used one or the other, and have used both at once when I needed even more than the Large.  If I were cooking even just a whole lotta burgers for company, the Medium is pretty small.  I don't cook ribs very often, but the Large is better suited for ribs.  I really love the Medium, but I'm really not sure I'd want it to be the largest BGE I had.  If I were choosing between only a Medium or only a Large, I'd probably go with the Large, even though most of the time its my Medium that I cook with.

I have never tried to estimate how much less charcoal the Medium uses, but if you just go into a store and visually compare the area and volume of where you'd put the charcoal on a Large or a Medium, it'll be clear that the Medium is a lot smaller.  But I have no idea how to translate that even into a guess on how much money one might save in one or the other.

I've never really paid much attention to how long they take to get up to temp, and I would have thought about the same amount of time.  The large has more volume and more ceramic to heat, but it also has a larger fire.  I might be wrong, but if there's a difference, I doubt it's huge.  With both of them I tend to light it and then set a timer for 15-20 minutes to go out and see how it's doing, adjust as needed.

And I've been resisting the urge most of us guys have to accessorize to the max.  I have a plate setter for each Egg, a Thermapen, and a Maverick (I think it is) thermometer with an indoor unit to tell me the temps, but no AR, no Woo, no spider, no bricks, no temperature-regulated blower fan.  There is a Woo ring for the Medium that will take the grid to the level of the opening.  I keep thinking about trying it because so many people here talk about raised direct, but I'm so happy with things the way they are that I just haven't ever quite had the incentive to order one.

And I imagine dealers are variable, but surely you'd have the right to insist that the dealer set it up right, and if there's an overbite, to fix it.  I've never had a problem with my dealer.  They delivered the Eggs all set up, and they were set up properly.

Theo

Have you checked to see if the BGE rain cap fits the KJ? I'm sure between the MBGE, LBGE and XLBGE one of them might do the trick. 
The medium grill dome extender is self standing, it only is a 13" grid diameter, with the splayed feet it is 16" wide so it will sit right on the fire ring of a MBGE. I don't think they make it anymore, probably why I got it for $15. There is a large, which has a 15" diameter grid - you could bend the feet in to make it fit a medium I suppose, 

I love my medium, here are the answers to your questions

1. woo/ with stone 10"
2. yes
3.yes, much faster.
4.yes, 5 lbs fits in mine when empty, and it will usually do 2 cooks +
5.no
6. slim.

Another XL/M owner here with only two of us at home. We bought the XL first and have no regrets as it is great when a big group cook is planned. We also use it for doing several racks of ribs or pork butts and then vac seal and freeze for those times when it is too cold, snowy or just plain inhospitable for outside activity in the winter. The Medium does get used more these days for the cooks for the two of us and works well to split a single rack of ribs and couple of yams. (My wife's favorite Egg meal.) You will like the Medium for two I am sure, but it is a little limiting if cooking full meals for many more than two unless you get creative with the space available.

I have a medium and an XL, recently I have only been filling half of the XL and using it that way for indirect. I bought the XL after the medium and my thinking was for group cooks I need more real estate. 

@Skiddymarker The two grills should have roughly the same efficiency based on the ceramics and sizes, but there are other variables in play - tightness of the gaskets, airflows, no ceramic cap after the cook is over, etc.  Like @SmokyBear I have the feeling that the KJ Classic burns up charcoal more rapidly.  I am always surprised at how much charcoal the KJ burns.

the Nakedwhiz.com has compared the LBGE and the KJClassic.  They concluded they are both equally efficient i.e. should burn lump at the same rate.

Just a guess, but if you want the medium to save on burning excessive lump, how much lump do you have to burn through before it makes economic sense to justify the medium? My opinion would to keep what you have, unless you sell to the neighbor.