(1) If you make over $61K or so over the year, you'd have to repay 30% of any EI you received during the year, at tax time.
(2) Your EI is still subject to your standard tax brackets. So if you're being undertaxed now, you'll still pay up at tax time (or get less refund). How much you get taxed now is kind of meaningless with that respect. It just gets us investing/speculating-happy people excited, who like to, with RFD tradition, like those extra cents time value of deferred tax.