Here's my understanding for the Canadian law:
Mind that I'm no lawyer, this is my understanding after some research I made on the web, and discussion with other fellow Canadians (Penguins and Eskimo included)
500 fps and 5 joules
As you understood I think, the limit is 500 fps OR 5 joules, not and. Which means you can fire a cannonball, if it is slowly enough
Also, it is not illegal, it is restricted. Which means you need to declare a airgun that doesn't comply with those limits, that's not a way to forbid you to own it, just a way for the government to know that you have a weapon in your possession that is considered powerful enough (whatever enough means by definition of the law).
For example, I had to register my Diana 35, because the velocity was higher than 500fps. Nothing wrong in that. I know have a nice Possession licence issued by the government.
(no no it's not me.. no mustache )
Of course, by doing so with a gun you made yourself, you will attract attention to you. The decision to register the gun is in your hands. It also depend if officials in your region are very smart, or very stupid. By default I would fear about the later, but you never know... Canadian civil servants are so much smarter and nicer than French ones... you surely have no idea I know them both very well now so I can compare.
Something that is illegal in Canada is a replica of a gun after year 1898. That means something that look exactly like a real "modern" airgun (including airsoft guns) is prohibited. That's more than restricted, it's forbidden to buy one. There is an exception to that rule, people that own airsoft replica before 19XX (don't remember the exact date), they have a right to keep them. But you cannot buy any officially, the customs will seize them at the frontier. You can sell them outside Canada though, weirdly enough, but anything that goes out the frontier cannot come in anymore
As for something you built yourself... it's very blury. You're not supposed to do that, you're supposed to buy it. So sale and import are forbidden and inforced. But if you made it (and I plan to do that too ), then who knows... I couldn't find any real answer, some day I'll call the Firearm officials and ask.
My advice would be... don't try to trick an official if you are "caught". You are supposed to know the law, and either the guy will be dumb and you will have the worst case scenario, or he will be smart so by trying to outsmart him... you will also have the worst case scenario