I would think you would loose a good bit of flow from louvering the y-pipe, as this would be the most restrictive part of the muffler, the louvers and packing material would get a ton of flow at that area? with the y-pipe louvered i wouldnt think you would get near as much flow to the other 2 straight louvered pipes, and it would cause a bit of a bottleneck there, just a thought..
I suppose you plan on trying it with a non-louvered y-pipe as well...