Heavy insulation mitten liners

Well the Hestra Primaloft Extreme Liners are extremely warm, but are quite bulky, so they would need a very big shell if you are going to use them in a shell.
http://www.hestragloves.com/#/en/gloves/liners/primaloft_extreme_liner/

If I remember correctly, they weigh 160 g (for the pair).

I believe the palm is 400g Primaloft, and the back of the hand is 600g Primaloft, so that is 10x the insulation normally found in apparel. There's a lot of loft.

I think it was on this site that I read a recommendation for Dachstein mitts, and got a pair — I haven't had them out in super cold weather yet, but my brief experience has been that they're too warm if anything, when other mittens are "okay". A bit on the heavy site, but worth a look.

Call MEC they had them this winter…
I think Goosefeet does a down mitten too or OR 400?

Ragged Mountain Tuckermitts

http://raggedmountain.com/index.php/hats-gloves-socks/mittens/tuckermitt.html

I have had Dachstein mittens since 1977 and they are great.

But my warmest liners are double layer fleece by EMS. (No longer made) But you can buy fleece at a fabric store and make them yourself. Just be shure your shells are large enough to accomodate them.

Or you could buy pile at the fabric store and use that for a single layer liner.

blackrockgear.com has 900 fill UL down mitts. if you didn't need insulation on your palm (for climbing or w/e) these would be an elite option.

Moonlite pile is one of the lightest and most hydrophobic fleeces ever made. When I hiked the AT in 1989 I had a whole suit made out of it by Moonstone Mountaineering, a jacket with huge chest pockets that could hold 1L bottles and a pair of side zip pants. No matter how wetted out my rain layer (Houdini Pullover) got I always stayed dry and warm with my Moonlite Pile underneath!