I have a 2012 Shimano M985 double crankset but I want to move to a triple.

It needs to be light weight, stiff, offer great shifting and have a 44T big ring.

Which companies should I be looking at? The XTR only goes to 42T and I am sure it isn't the lightest.

The options from Tune and Extralite look interesting but I wonder if they would flex a lot compared to the XTR?

Try finding a 2011 Race Face Next SL 44/32/22. The 2012+ version is 42/32/24.

609g without BB (82g lighter than the XTR triple). It is as stiff and shifts as well as the XTR.

Race face next is a good option. Other are Cannondale hollowgram and THM Clavicula.

Chris.

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MSC Koncept Carbon Di2 - 6955g
Leichtkraft Team Carbon - 6868g.

Clavicula is super-high-end, but IIRC, the mtb version is about 166 Q factor.