If it remains significantly concentrated on and just beneath the surface and hits the area with 40% of the nation's marshlands, this is not true. Location, depth and currents will cause much more damage than Ixtoc.
I'll grant the significance, if any, of plumes remains to be determined.
I can't for the life of me figure out how you reach such a conclusion. The depth of the spill and the use of dispersants means that a much larger fraction of the spill will be "scattered" (dissolved, dispersed, whatever)before the spill ever REACHES the surface (compared to Ixtoc. which was almost totally confined to the surface).
The dominant current in that area flows west-to-east, so again, less of the spill should hit Louisisna. And by the time it gets to Florida, the nature of the spill will have changed (weathering, evaporation, etc) to more nearly match the state of the Ixtoc oil as it reached the Texas coast......which, due to the Bureau of Land Management study done on the Texas coast, we already know the impact of.