Let's play hypothetical:
You order an AEG which is actually designed, labelled and capable of shooting in excess of 407 FPS. Currently, the only AEG's capable of all three feats are the Systema Max PTW's (we will discount BA rifles for this scenario). If you imported this item, supplied the proper paperwork claiming it's veloctiy and this could be readily verified by a customs agent just by looking at the box or manual, and seizure was enforced claiming "replica firearm", you'd have very good grounds to win an appeal.
You have to play the appeal game with customs by their rules. If you can prove that the seizing agent violated either the law (unlikely) or the CBSA internal guidelines (available on-line), or gave you conditions for release that you comply with, then likely you will be successful. I have fought and won an appeal this way.
Now, if you were to try this with a CA, Marui or similar AEG, where it states on the box or in the manual that it shoots 90 or 100 m/s instead of the 124.5 that customs uses as a benchmark, then you will not be successful and your gun will end up as a blob of hydrocarbons.
The appeal that I won still ended up in gun destruction as I could not prove the velocity, but I did prove errors in the seizure and conditions for release were met at the time of the initial appeal, but by the final decision those conditions (license) were gone.
You have to play their game by their rules (not necessarily the law), and know and prove exactly what you're importing. That is the only way to beat them.
Age verifier Northern Alberta
Democracy is two wolves and a sheep discussing what's for dinner.
Freedom is the wolves limping away while the sheep reloads.
Never confuse freedom with democracy.