For those folks who don't know what "LSA" means, or for that matter, "Lobe Separation Angle", here is a graphic representation of the two cams at a typical 108˚ from Top Dead Center:
The separation angle determines the degree of overlap in these two curves.
And to explain "why" LSA is important (using that awesome graphic) you have to remember in a boosted environment you are actually cramming your combustion chamber full of air under pressure. If you have overlap (that small section where the two lobes cross over in the graphic, this is actually when both the intake valve and exhaust valve are open at the same time), what you are effectively doing is cramming all that boost right out your exhaust. You don't want overlap in a boosted environment as it is a giant boost leak straight out your combustion chamber into the exhaust right when you are trying to fill that chamber with fresh air.
You want that overlap in a NA environment because you are in a negative pressure state and it creates a scavenging effect which effectively sucks more air into the combustion chamber. At that point the exhaust pulse has gone down the runner of the exhaust manifold or header, and has left a negative pressure zone behind it. That negative pressure zone is under vacuum and effectively sucks air in from the intake manifold during overlap. This is also what gives that lope-y cam sound we are familiar with.
LSA is a massive influence on all engines, it just works differently (and oppositely) when you are in a N/A vs. Boosted environment.
So, why is it that people have gravitated to lift being good for boosted engines and not duration? I'll explain with that graph again. See the tiny area of overlap between the two lobes? Well if we increase lift we don't impact that area at all. We just take those existing lobes/peaks and make them higher. We have effectively increased the area under the curve WITHOUT adding any LSA (which would be an effective boost leak in our engines). Now if we increase duration we are making the lobes wider. Well, if we make the lobes wider we are decreasing LSA, adding to the boost leak, and losing some benefit of cylinder filling under the curve.
BUT!!! We have full VANOS control!
What does that mean? That means in theory we should be able to run cams with increased duration IF
we completely remap all the VANOS tables and retard the intake cam a bit and a advance the exhaust cam a little. That would effectively move those two lobes apart on that fancy diagram, thereby removing the additional overlap we induced with increased duration, and giving a greater LSA.
So the old adage that adding lift is better than duration for a boosted engine was on old statically timed cams. An environment where adding duration would inherently add overlap (which is bad for a boosted engine).
We don't live in that world with VANOS, it would be a completely new thing to re-tune all the VANOS tables to take advantage of CAT cams...but it would be possible.