The obvious example would be evidence that when the various prosecutions of right leaning politicians started, membership of their organisations trailed off or the growth rate slowed. In the relevant countries at a minimum.
You actually didn't explicitly state it was your opinion. This conversation was started because you did the opposite. You explicitly stated it was the correct way to do things and was objectively a perk of being an American, only later when I challenged you on it did you state that it was your opinion. But that's not overly important, and I know what you're trying to convey. And this is why I find your argument problematic and what I've been trying to get at.
1)We agree that there can be negative effects to free speech
2)We agree that there are certain effects so negative that free speech should be curtailed to prevent them
3)We agree that, painting with broad strokes, Europe has greater restrictions than the US
I think at least?
But then you go on to say:
4)Those lesser restrictions in the US are, in your opinion, a positive to the US
5)Countries which have greater restrictions you, therefore, find less desirable
6)One of the ways in which they are less desirable is that you imagine there is less free political discourse (though, apparently, can't think of a way to prove it)
7)Therfore (4) is correct.
If we assume there is a negative to allowing the WBC church to exist, all else aside, and also accept that European laws would criminalise them while US laws don't then you need to show the good that is served by allowing them. The opinion of many americans doesn't matter. Many americans have been wrong before, they will be wrong again. (Obviously I'm not singling the US out here, just that thats the country we're talking about).
The only thing you've suggested as a perk that outweighs the harm hasn't been supported at all.