They probably won't ask for it at customs, unless you get ramp checked by transport Canada. In fact, they didnt even send a customs officer to meet me in Kelowna where i entered Canada, nor did they ask about insurance on any of the online apps/forms or multiple phone conversations.
If you are unlucky and do get ramp checked, I'm sure the trans. Canada feds would then ask for it, and if you don't have it they could impound your aircraft and/or fine you thousands. I have no idea if they'd do that just for wrong limits, but I'll bet they could if they wanted to. A lot of Canadians do not hold US citizens in high regard, and I'm sure there would be no shortage of government officials there who would love to take down some US based pilot and put them in their place. I bought liability insurance for my airplane just to fly it through Canada while I ferried it to where I live in Alaska. Even the liability only policy cost me $3,000, but I couldn't accept the risk of getting caught, not to mention I comply with all regulations, even obscure ones, when I know about them (even the financially painful, and in my opinion stupid ones like this).
I was flying a cub solo through very remote areas, the odds of me damaging anything other than a tree or a rock if I crashed was extremely remote. I still bought the insurance. Then again I drive around town in my car at the speed limit, and carry insurance there too, dodging the hoards of uninsured speeding maniacs, so maybe I'm the weird one.