Well, I received my reply from XM Canada and it was about as unhelpful and generic as I figured it would be:

The recent merger between XM and Sirius in the US has allowed for some additional best of programming to be available at an additional cost. The best of packaging, and Tiered Pricing is currently not available for XM Canada or Sirius Canada subscribers. We will ensure that any future changes to our line-up or availability of additional programming is communicated to you as soon as the information is available. Please visit our website at xmradio.ca for up to date channel line-up information.

Hmmm, 'currently not available'? Leaving the door open or false hope?

I have two receivers due for renewal in mid-November. So I'll be giving some thought to going grey. If the 'Best of' options included NPR and CBC, I'd bite for sure. But also knowing that the option for them to add and tweak their best of packages somewhat compels me to try XM USA and see what happens. Hell, I'd enjoy listening to Coast to Coast once again!