After reading through pages and pages of listing, and tax rules, this is what I got. It only matters where a company is registered, not what exchange it trades on, so If POET stays registered as a Canadian company even though it was to trade on the NAS, if your American shares were held in a Canadian account then if you sold American shares the money would be converted to Canadian currency and deposited into you account without the normal 15% withholding tax.

If POET was to become an American company and a Canadian held shares then any sale of shares would be subject to the 15% withholding tax, and as a Canadian resident you are not abe to apply to get that tax back.

It's a bit complicated.