I think he's coming from Canada which should be a lot easier than the UK from a customs perspective.
10kg is definitely doable but you're right, requires a lot of squashing. We got 10kg each into 2 bearvaults for 180 miles of PCT through the Sierra from Kennedy Meadows to VVR. Despite this our daily calorie deficit was greater than a normal calorie intake. Pretty much everything in the can would sink in water (e.g. spaghetti, instant mash, couscous), and it was squashed very hard. I think the following year, for Mammoth to WP we got about 8-9kg in each can.
At the end of all the packing, if you can't get another kg of rice, M&Ms and skittles (poured loose) into all the gaps I'd be amazed, but very few people seem to do this.
My test pack of a can last weekend got 6kg/13.2lb in without any aggressive squashing (just careful tessellation), nor letting the air out of the packets, nor pouring in loose granular items.
Packing a bear canister was my intro to packing a bear can and worked for me.
