I'm sorry, I somehow overlooked that post. Please let me reply below:
1. No, I wasn't implying Overkill sells more in the US than Testament. I only said that a bigger part of their album is sold in the US than elsewhere, unlike Testament which sells a lot outside of the US too. This doesn't mean that Overkill outsells Testament in the US. I only referred to proportions not total numbers. For example if Overkill sells 100.000 copies of an album worldwide, 60.000 of which is in the US that doesn't mean that they outsell Testament which sells 400.000 albums worldwide, 160.000 of which is in the US. It does, however, mean that a bigger percentage of Overkill's albums is sold within the US (60%) than Testament's (40%). Obviously I've just made these numbers up as an example.
2. I've never seen those numbers anywhere. Do you have any links to sites where such amounts are mentioned. They seem completely out of place in my eyes. Why didn't they get any gold albums in any country if they sold 20 million albums worldwide. Besides, we all agree that Testament sells much more than Overkill, and Eric mentioned last year they only sold over 4 million albums (I don't know how accurate that is but it gives you a perspective). There's no way Overkill sold 5 times more than that (They would have been in line with Slayer and Megadeth and ahead of Anthrax in terms of popularity, which unfortunately is not true).
3. Here's the link to the OverKill forum:
http://wreckingcrew.com/yabb2/YaBB.pl.
As for the Megadeth board - I've never been there. It was Sole Reaper that mentioned it (
http://www.ultimatemetal.com/forum/testament/334366-has-nuclear-blast-ever-had-band-sell-gold-record.html#post6520561). I guess he could help you with the link.
Should you get any interesting info on any of these, please feel free to share it here.