What is your shotgun chambered for..?So are the plus 2 extension tubes illegal or is it only illegal to install them on the Canadian side?
Do the math, but be careful... I'd go by the published SAMMI specs keeping in mind that the overall loaded length of ammunition does vary slightly between manufacturers.
As a side note... For competition/action shooting people get really hung up on capacity... If you actually learn how to shoot, move and load having that extra round or two in your mag isn't going to matter.
If you're REALLY hung up on capacity... Buy a pump.
Pump guns are not subject to any magazine capacity restrictions so... On a tube fed gun you can have as many rounds as you want.
Of course... Like I mentioned... Unless you put the time in and practice you're still going to get your ass handed to you by a guy with his 5 round semi who knows what he's doing.