From my experience and observation, when all attributes are 20 and the driver is mimimum weight they max out at L19 in the current version of the game.
When the new game was launched in August 2016 some of the drivers that were migrated from the old game came across as L20 but I imagine all these drivers will have retired long ago unless they're locked up in inactive accounts.
Many months ago (I can't remember exactly when but was a good while back) I seem to recall this very issue was raised on the forum and the devs ran a script to update drivers with all 20 attributes to Level 20. I expect some of these drivers are still knocking around, here is an example of one which was updated by the script...
Lewis Hamilton
He was a L19 before the script was run then overnight he became L20 but he was no faster or better than when he was L19.
So in a nutshell, it's like Boomer says, providing all stats are max there appears to be no difference in performance between L19 and L20.