The 7 degrees is all me. I've been researching late model cars and found the S197 Boss 302 had 7 degrees, plus others. @72bluNblu mentioned once (if memory serves) that he ran 7 degrees for awhile and then dialed it back to 6.
So I have run as much as +8° caster on my Duster. Currently I’m running +6.5°, which is pretty close to the tipping point for the steering becoming much heavier with turning.
With +8° the steering was noticeably heavier, and the jacking effect from the positive caster was very noticeable as well. I did run the car for quite awhile like that though.
The steering gets quite a bit easier dropping below 7°, but dropping below 6° with the 275’s and the tracking issue starts to become noticeable and the steering doesn’t get all that much lighter.
So I generally set about +6.5°. No less than +6° and no more than +7°.
That upper number though has a lot to do with my 16:1 manual steering! With power steering it wouldn’t be as much of an issue, although I do think the jacking effect would mean you’d want to keep it under +8°.