This topic raises its head from time to time. Coaches who have coached both are unanimous that it would not be close, and in favour of the CIS. (Harry Neale, Mike Keenan, and Dave King come to mind, and they also coached in the NHL.)
The biggest problem for a junior team would be the 16-17 year olds and their 3rd/4th lines. I would point to world juniors where an "18 year old team" usually disappoints
vis-a-vis a "19 year old team". At that age every year makes a big difference. The yearly change dissipates as one gets older.
Bob Stauffer once posted about the last known exhibition game, where the Alberta Evil Monkeys beat the Edmonton Oil Kings 10-1 or something wild. Thus ended the odds of such games in the future.
In world junior training camps there have been a number of games. (See at Wikia:
Canada West Universities Athletic Association All-Star Games.)
In my estimation, Team Canada Juniors are at the level of the
creme-de-la-creme USports teams (Alberta, UNB, and the occasional interloper). Other medalist countries are at the level of the good teams (Saskatchewan, Manitoba, Calgary, St. FX, St. Mary's, Acadia, McGill, UQTR, and whomever is having a good year from Ontario).
The Team Canada Juniors are a mile or two above a club team in any CHL league. At one time the defending Memorial Cup winner represented Canada. That stopped when they were finishing 6th or 7th.