Simple answer - No. Lords on war islands shouldn't need to worry about tax changes.
War Islands runs a bit differently on some things. It does have an acceptable base tax rate, but it doesn't have other anti-large realm penalties because well, its counter-productive to the island's purpose. So yes, its getting the new changes. But, to counteract their increased effectiveness, its starting rate before realm-specific mods (what was said as 10% above) was increased above stable/testing. So the end result should mean that War Islands should be unphased, despite getting the new changes. A smaller realm will be able to run higher taxes on war islands than a larger realm, but the larger realm won't be as hurt as a larger realm on another island.