the idea of the noisy flap is quite sensible - setting it open during idle was a nasty childish idea - but it is in many ways is just shift cam on a budget - fit lively cams trying to get it to breathe and make serious go at the top, and lower down the revs half the air falls out the exhausts - so to compensate you have to ride it worse pushing up emissions - bung up the hole and it works better ...EXUP YPVS ATAC all the same basic idea try to keep the fuel inside where it makes a better bang rather than just a smell
I do not see the relevance of the systems that you have mentioned YPVS was used on 2 strokes !! EXUP altered the collector volume so was close enough to the exhaust ports to effect the cam overlap, and Atac was used on the NSR500 not a plodding old boxer engine.
I still feel convinced that it was introduced for sound emissions as it was after the cat a long way from the exhaust ports..
I do not see how a valve introduced after a restrictive catalytic core could have any bearing on cam overlap.
Regards De-cat headers and stock exhaust, it is a good budget option, I have a friend who uses this , louder than stock but not as loud as my full Remus system.
Here is a photo of a couple of cat cores that I have removed in the past just to illustrate how restrictive they are.