From my own experiences, I’d say more than 1 degree was too much. I initially set mine up for negative 1.2 and while it provided really good turn in, it was just too much for the bumpy Scottish B roads I was mostly driving on. The car was wanting to find it’s own way over all the bumps and stuff. I also found it scrubbed the inside edge of my tyres extremely quickly. I’m now running at just under 1 degree negative which is more manageable.
I don’t suppose that helps you very much, but to quote Jimbo; anything more than negative 1.2 on the road will be too much.