Simulation vs. Arcade
I would be interested to see a similar study done using software that attempts to be a simulation rather than an action game (eg. rFactor, LFS, GTR2, etc.).
I used to (and still do, to some extent) play these simulators and I do believe they have improved my real-world driving. Perhaps not so much the actual physical coordination, but general theory and above all my concentration.
I can see that it would depend a great deal upon the mind-set that you approached the simulator from, but if the intent is to practice for real-world driving there can be a significant benefit.
For instance, how many people know the correct way to react to oversteer/understeer situations and how the required response differs between front-wheel-drive and rear-wheel-drive.
I understand that was not really the particular aim of the study, but that being the case, what is the difference between an action racing game and, say, Sonic the Hedgehog. Perhaps they ought to try the study simply showing clips of car-chase scenes and other action-scenes and compare the results.