Very real. It's why Stamkos didn't take Toronto's much higher bid and chose to stay in Tampa instead. If you don't think taxes have an impact on effective caps of every team then you aren't paying attention.
You do realize that
1. Most players if they don't pay state taxes for half their games (they pay state taxes on road games) have to pay more in their federal taxes.
2. Cost of living differences in Florida eat into the lack of taxes.
3. The way things are structed in the NHL most players pay a lower effective tax rate than us regular idiots.
4. Because of their wealth they pay a lower effective tax rate than most of us because they have access to tax professionals.
5. To put a finer point on this: They only don't pay it on home games, they pay taxes on road games.