I don't think the clutch has anything to do with it. What I do know is that on many games, like GTR1, Race07 and rFactor, I am usually competitive with most mods at 110% or 120%. With GSC, except for rare instances, I have to set them at 100% and in many cases that's too fast for me too.
FYI, I love to use my Gated shifter and H&T whenever I can, I was just racing the AI in GSC at Donington with the DRM mod. Lots of fun, but definitely, I am slower when driving "properly". Even with the H shifter but autoclutch I am about 1/2 second faster on the average lap.
I just cannot bring myself to drive vintage cars with paddles. It seems unnatural to me.
As far as 120% needed some people to be able to beat it to justify its existence, yes, I am sure there are people that can beat it. We call them Aliens. And that's why we call them aliens.
A couple of weeks ago one of those UberAliens showed up in our league (which already has its share) and absolutely beat us all mercilessly. I usually finish in the top 10 (but rarely the top 5) ad he passed me as if I were changing a tire by the side of the road. A couple of our best guys gave him some hard time, but eventually he won every race he entered.
He is a pro driver IRL, but not a world famous one, mind you, and currently he is racing in the minor leagues still.