To my mind and from my experience and various actual driving courses. etc taken over the years the issue lies in the stock, non adjustable seat being to laid back, as in your bum goes too low leaving your legs unsupported.
I prefer the over the road trucker position, some might call it the bus driver position. Anyone who drives long distances at all will be sitting upright, fully supported, at the wheel.
The seat as designed makes you lounge, sitting back as if you are a rapper in an Escalade or something.
What I did, years ago now, was (welll, I watched as my mechanic and the Norton bike restore guy actually did it) was loosen the front seat bolts and remove the rear ones. They used some square stock to make spacers but as Jack points out fender washers will work.
I'm not certain but my rear seat attach points are up at least 1.5", maybe 2". Then everything was torqued back down which "moved" the front brackets a bit as they snugged up but I sit far more upright and far more supported over the entirty of my seat, legs.
Just make sure you don't put any spacers/washers over the carpet and there are little guide pins on the stock seat brackets you want to ensure go back from where hey came.
Even tho I only watched (and provided the beer...) it was a quick, simple job that I wouldn't hesitate to do myself...and it immediately cleaned up my issues with the driving position.
The other thing is really helped with is the arm on the open window sill stuff...new cars have such high belt lines (for safety I guess) that its actually uncomfortable for me to do that arm out the window, summer drive thing because its at an upward angle...my seat lift mod cured that too...level on the level and now i look like a guy driving a CUV, not some rapper rolling dirty. Or however the kids say it.