Is there a benefit to buying The Witcher 3 GotY if I already own the standard version of the game?

No.

But why not read the official announcement: http://forums.cdprojektred.com/threads/119161-Release-date-announced-for-The-Witcher-3-Wild-Hunt-GOTY-Edition?p=2814811&viewfull=1

You get some extra lollies:-)

I would have tought that they'd atleast offer an upgrade deal for us...oh well.

That makes no sense at all. There's nothing in the GOTY that the standard package doesn't contain. If they offered an upgrade deal, you'd be paying for nothing. And if you don't own the DLCs yet, your upgrade deal is the season pass. You're getting the exact same content in any case.

It kinda makes sense. If you bought the game at release, you paid between $50-$60 USD for the base game, not including the $24.99 cost of the season pass. People buying the GOTY edition are getting it for $50, which in itself is $5 than buying the base version plus the season pass at the currently listed prices. I think it is reasonable for those who purchased the game upon release to expect a discount of at least $5 on the season pass.

On the other hand, those of us who have been fans of CD Projekt should have expected a cheaper collection of all the DLC within a year of the games release. Personally I'm kicking myself for not waiting until the "Complete Edition" came out, a mistake I do not plan to make with Cyberpunk.

That release post on the official forums says no new content is being added to the GOTY version, yet the GOTY Edition's purchase page says: What exactly is that 'new user interface' it mentions?

The new interface came with one of the patches.