That's the figure that researchers from the Georgia Institute of Technology have concluded upon. The obsession of BP for reducing damage costs by means of hiding the oil and the permissiveness of the US authorities with this destructive behavior made the resulting product (Corexit dispersed oil) to be 52 times more toxic than would have been oil only.
Dispersants were certainly never used near the coast in European oil disasters, instead heading towards the oil with rudimentary means and capturing it in open sea, before it could reach the coasts worked reasonably well in the case of Galicia - being of course a popular idea and initiative and not the design of any authority, much less greedy heartless corporation.
Source: Science Daily.
Ref. Roberto Rico-Martínez, Terry W. Snell, Tonya L. Shearer. Synergistic toxicity of Macondo crude oil and dispersant Corexit 9500A® to the Brachionus plicatilis species complex (Rotifera). Environmental Pollution, 2013; 173: 5 DOI: 10.1016/j.envpol.2012.09.024