...this watch is a winner! It fills the gap as you suggested. A gap which should never have been there in the first place. Because when you think about it, the Master Control line has always had the Master Moon, the Master Perpetual and later the Master Calendar. But this complete calendar was not to be found in the Reverso line for reasons unknown.
I agree with your assessment of the smaller movement relative to the case. Do you have more information about the Caliber 843? I wonder if the base
movement for it is the same as movements found in the GT sized Reverso.
The bridges layout suggest so. But I should not make any more guess
lest I will make a fool of myself 
At least JLC has done well to deceive the eyes with the elaborated frame that goes around the movement. Besides, it is much more elegant than my 976 as shown here...
Whatever they do, don't close the back with metal! That beauty of a movement deserves to be seen despite its small size. Display backs mean even more to me these days than before I think.
Speaking of size and the 976, I quite like the presence of the 976 case on the wrist despite the fact that it is say 15% too large for my wrist. I find its proportion more refined than the XGT and the size more contemporary than the GT. But I know this is a very subjective matter and everyone has their own preference. I'm just saying that it works for me and that it allows the watch to be worn more easily in more casual circumstances.
Between these two I think the rose gold looks much sweeter. But as with the Grande Sun Moon I already have, the only remaining rational part of my brain cannot justify the difference in price between the SS and the RG. This Grande Reverso Calendar is no exception. I have checked with my dealer and found that the difference in retail prices between SS and RG is easily the retail price of a stainless steel Memovox on strap. Personal financial status dictates that SS is still the way to go. Besides, we haven't seen Basel yet. If Omega is to release something with a moonphase, your friend here will most likely need more $$$$ again.
Nicolas, thank you so very much for giving the peace of finally knowing for sure how the back looks. These specimens you show look neater than the ones at SIHH also. I can see that for one thing the the discs are properly aligned with the windows now. And they are perhaps more flushed against the windows too, judging from the almost lack of shadow on the discs this time.
I am looking forward to wearing the GRC already.
Ruckdee