Sorry pro-gamer is incorrect, it is dependent on the mobo if 1.0 will work with 2.1. 1.0 has no issue with 2.0 but there are 1.0 mobos that don't support 2.1. And there really isn't a performance decrease as the cpu on the 1.0 mobo is more than likely the bottleneck.
As jaguar and wa1 said in the post you linked to, sometimes it may not work which proves my point. Also compatibility has nothing to do with performance. 2.0 x8 is the same as 1.0 x16. Even high end cards such as the 570 and 6950 only show a 2-5% decrease which is unnoticeable in actual gameplay as it only equates to 1 or a fraction of a fps. http://www.tomshardware.com/reviews/pci-express-scaling... The lower end cards will show no change at all.
I've helped plenty of people where 2.1 doesn't work and I've helped plenty of people where it does work. Yes it's true it's a minority but it still happens.