Getting the weight down on your bike is a slippery slope from here on in, but I'm with you on not compromising function
You're definitely heading along the right lines by getting the weight of the wheels down. As you've stated in your other thread, Hope hubs are worth looking at for being good value and not too heavy. It depends on the type (and weight) of rider you are, but a well built set of Mavic XC717's or DT 4.2's should be pretty strong with DT comp spokes - maybe you don't need to suffer the extra weight of the 5.1's?
I don't know much about SDG stuff, but I thought it was quite pricey. Its not very
weightweenie, but I use a USE carbon post and a WTB Rocket V ti saddle. Dead comfy and nice and strong.
With the the brakes - you should find all the stopping power you need in Hope Moto V2's, with less weight than the M4's
IMO, XTR mechs don't offer much of a weight saving for the difference in price over XT, but its certainly something you can upgrade to if you have the cash...
Another good place to look to save some weight is the tyres - I use Nokian NBX 2.3 on my Nicolai, which are really quite light for the width and offer good grip on rocky stuff, which I figure you have a good bit of in Scotland
If you want some more racey (read puncture prone), look at Schwalbe Nobby Nic's in the 2.25" width.