Whats wrong with XTR caliper - they fit a 140mm disc (Shimano have a compatibility guide here). The weight Ultegra is 123gr, of XTR M9000 is 91gr
Disclaimer: Brakes are a safety critical component, I do not recommend mixing road and MTB parts.
According to the link above, all MTB calipers/levers are compatible, as are all road lever/calipers combinations, but the guide does not mention cross compatibility. Many people on the net (I have never tried it) claim they are interoperable. Apart from bleeding, if the piston diameter is the same I see no reason why they would not work. If the piston diameter is different between the road and MTB setups then there will be differences in performance as the amount of fluid to move the pads the same distance is different.
With some of the newer discs Shimano has changed the bleeding systems. Its possible a lever/caliper mix might be hard, or impossible, to bleed.
The hoses used for these brake components are the same (BH90) so there will be no problem connecting them. (Even if tone was BH59 and the other BH90, the outer diameter and olive is the same so they can be mixed and matched). The only thing to watch is if the caliper end of the hose is a different fitting you may need new hoses.