The problem with the wake separation is that is not an exact science but the FAA makes it seem like it is. IE a heavy behind a heavy needs 4, a small behind a 757 needs 5, etc. There are much more factors involved, but to simplify things the FAA set these numbers in stone.
Some of the other factors include:
1 winds. If there is a 30 knot crosswind I'm not too worried about the guy I'm following, I might however be a little concerned for the guy on the upwind parallel that is a couple hundred feet higher than I am due to his threshold being farther down from where mine is.
If there is a strong headwind the wake should be no factor.
If there is a direct tailwind you are going to need extra distance or stay above which produces the problems already listed.
2 weight/acutal weight vs MGTOW
My MGTOW is around 45,000 lbs, a 737-900's MGTOW is more than 174,000 but we are both "large". Also a 747-400 on a ferry flight could be 200,000 to 300,000 lbs less than gross weight. Also a 747 needs 5 miles behind a 757 and that 757 needs 5 miles behind the heavy. Who do you think will be affected more by the wake? The 255,000 plane hitting the 800,000 lbs plane's wake or the 800,000 plane hitting the 255,000 lbs plane's wake?
3 performance. This really only deals with departing/go around aircraft. Some planes are just pigs. I have no problem departing behind an A340-200 because I am usually above him a couple of miles after departure. On the other end you have a 757 or a lightly loaded heavy. His departure path will be much higher than mine posing a problem. Like I pointed out above I am 1/4th the weight of aircraft that are in my category. I can get assigned "maintain visual with the A320/737 cleared for takeoff 24L" when the other plane is still rolling down the runway. You don't assign a 737 to maintain visual with a 747 and clear them for immediate takeoff while the heavy is still rolling down the runway but the ratio of weights is about the same.
The worst wake I have encountered was from an A319 that was very lightly loaded. He climbed like a rocket we got an immediate takeoff behind him and hit his wake at a couple of hundred feet putting us in a 60 degree bank. If I hit that wake at rotation it could have been deadly.
This really isn't inteded for you guys in the Northeast this is a Socal issue. The guys in NY and ORD are some of the best controllers in the world.