Oh boy...and so it begins.
I only saw the Master Control JLC before - wasn't too fond of the bezel/dial ratio, the arabic numerals and the date circle. The ultra thin, however, just happen to not include any of these pet peeves. Will seriously take into consideration the grand ultra thin - IMO the cool hour markings suffice to make it interesting, and I'm already stretching my budget (but quite fond of the geographic as well). We'll see, though, and I'll definitely take a stroll around the JLC ADs.
Edit: As an aside, I've yet to find anything on watch sizing, other than 40 mm+ = not dress. I'm quite tall, around 6'3", but wrists are around 6.75". Is there a rough guideline, or should I just eyeball it?
The simple answer is "try them on". I can't over-emphasise how surprised I've been by the difference in my mind and on my wrist, in how a watch looks. I always wanted a IWC Portuguese. Since trying one on, I realised it didn't sit right for me as a dress watch, and wasn't laid back enough for casual. Not quite. I still want it a bit, but I know it's not perfect. Conversely, I assumed Panerais would be just too big and comical, but I tried on a 47mm Radiomir and found it totally wearable. Depth, but also the shape of the lugs and just the aesthetic proportions, make a massive difference to how a watch sits with you.
Also, I'd suggest that if you normally wear a shirt/suit for work, then wear something similar when you go watch shopping: how a watch sits with your cuffs is really going to make a difference to you as an every day dress watch.
Having said all that, if you do like the Grand Ultra Thin, it has a lot going for it. For a 40mm watch it sits, as the name suggests, very flat and smooth, so it won't look chunky at all. I am a big fan of small seconds dials on a dress watch, and this one has great proportions. I would personally like some more texture in there somewhere, but given your wish to balance your height and slimness, these could be the perfect proportions for you. And it's definitely a proper dress watch. Go put one on, now!