Wsherif you make an interesting point.
While there has been intensive research regarding wake turbulence, particularly on final approach, there are instances when the "minimum" standard isn't enough (particular wind conditions etc). And as there are 5 wake categories in the UK, maybe there will be the introduction of a 6th?
I saw a nasty little problem not that long ago involving the at the time new A340-600 being followed by an MD-80. The following was about 7.5 miles behind the heavy, and by the sound of the R/T transmission signalling the pilot's intent to turn out of the wake, things were not all rosy on the flight deck. Of course, our seperation standards are a minimum standard, and it says in the book about being increased if deemed necessary.