actually there have been several threads debating that topic. Are the later SVT UIMs actually better then the 98's? The reason for the debate is because the later model SVT's produce a couple of extra horsepower, and many believe its because of the UIM. I'm going to have to disagree with them, as I recall there was a thread on either here or ceg.org where someone actually contacted the owner of extrude hone, and he called back leaving a voice message, which was then posted to the web, saying the extrude hone process was exactly the same between all years of the SVT, and that the hp difference had do with something else. I don't believe anyone has flow benched the UIMs between the different years to compare, but I seriously doubt there is a difference.
Now there are SVT UIMs out there that are better then stock, as many people sent theirs back to extrude hone about a year ago as part of a group buy to have the process performed again to maximize the flow, these have been proven to be better. But these would be hard to find, as I doubt anyone is going to be willing to give it up.