I have been researching this topic for too long, i have come to some conclusions.
http://www.billlawrencereview.com/BillLawrence_Timeline?q=node/13this website explains the differences between 2 pups well. from closely looking at nuno's N4, the pup is not filled with the epoxy which made me think that it was actually built by bill & becky, however, the blade is as thin as USA version, and bill never have produced pups with thin blades over the years. also nuno's pup does not have curved blades either. This led me to a conclusion that he uses an usa version. it is also confirmed that his paduck guitar has usa version.
http://www.flickr.com/photos/calmante/2408717058/sizes/o/http://www.flickr.com/photos/jonnywhite/625072080/sizes/o/A/B test (washburn n4) from the nuno forum can be heard from the link below; 1st usa 2nd bill & becky (not the best sample but u get the idea)
http://www.box.net/shared/jmkefl5k4wFinally, bill & becky pups stopped using ceramic magnet and it was substituted to alnico v magnet. bill claims that they have made some adjustment in their winding to produce the same sound as the previous ceramic pups did but I think it is total BS. think the sound is nowhere near nuno/dimebag territory.
I like how usa version sound. it may be a bit shrill for some but it sounds clear, dynamic, punchy.. it has character. it definitely fits guitars like N4s... it has downside also, like its build quality and weak bass etc etc... I wish Tim could make some adjustment and bring out better version of L500. :)
on final note, I found coil tapped miracle man can sound very L500 alike (check out my youtube vid for the clip)