Currently each hand at an online poker room is raked between 2.5% and 7.5% (until you get to significantly higher stakes, where the majority of players don't play). so 8% isn't the oddest proposal.
However, 8% is far too high, especially since the pokerroom will still need to rake the pot to make their own profit. What woudl work is either taxing deposits 5-8% or taxing each pot ~1%, maybe 2%. Anything over this will crush the online poker world into a point where the biggest rake generators won't play any more.