Interesting caveat if that is the case, but if it is then you essentially torpedoed almost any argument for many veteran free agents seeking a major contract. That would mean that Canadian teams have a leg up seeking veteran players than had previously played in the US and inherently invalidates the potential argument for making taxes something that could be account for in the salary cap if there are factors like that at play. And if there's a loophole like that in place, then there are almost assuredly other loopholes in place as well to minimize tax payments for other circumstances as well.
Here is the info on Tavares taxes.
The more I have looked into this the more I am convinced that yes, taxes for sure play a factor when a few percentage points can equal millions of dollars over the coarse of a 7-8 year deal.
However, the Tavares situation is also a prime example of why a standard policy would be almost impossible simply because there are WAY too many variables to account for because of all the tax loopholes available to players based on two countries, many different states/provinces, working in every state/province throughout the year, offseason residences, front-loaded/back-loaded, bonus vs salary, etc, etc.
At the end of the day, the priority should be on improving revenue sharing instead of worrying about taxes. Because taxes should only play a role if all other things are equal. Therefore, start by making all other things equal and then worry about taxes.