Hi, I think my answer is too late, but I had the w3x4 and I can say that personally, I did not enjoy them at all. The w3's beautiful V-shape for that energetic punch disappeared and became more neutral towards a w4, however still with a fairly bright sound. Less bass and less treble sparkle. Although the mids on the w3x4 have definitely improved in clarity. I think if you go with quads (w3x4), the w3's sound signature is lost. I think that if you like the w3 sound, just get them re-shelled and make them look beautiful but keeping the original sound signature. I am sure the extra isolation will improve sound quality by a bit since customs will create the perfect seal and fit. I have not heard the w3x6 or x8, so I can't give you much information on that, sorry.
However if you are considering on getting w3x6 or higher, then I would recommend to get the UM Merlins instead, if you can afford them. If you like the deep bass, then the Merlins are the best pick. I honestly don't think it is worth it to get any upgrade on the w3.
My w3 has been tamed. I am curious to know what you had decided to do!