Will an MTB hydraulic brake lever be compatible with road hydraulic brake caliper? I'm looking for a brake caliper that will be compatible with an XTR brake lever. As I browse through Shimano's line of lightest brake caliper, I found that Dura-Ace is the lightest but it's a road type brake caliper. So I'm asking,

Will an XTR hydraulic brake lever can be compatible with Dura-Ace brake caliper?

I'm thinking to pair XTR BL9000 hydraulic brake lever with Dura-Ace BR-R9170 brake caliper and pair it with RT-MT900 140mm disc brake rotor, if possible.

I don't do intensive downhill or anything but this is for my sleeper bike. So the highest/lightest component available is my option.
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.