Mmmm! Very strange George!
Having the 450, with a standard length clutch accentuating arm, I need a good grip at the best of times and have similar amounts of free play at the handlebar clutch lever and its pivot as yourself when cold, 2-3mm.
You would have thought that heat would have expanded the clutch accentuating parts and closed that gap, rather than increasing it?

So, it would appear that the clutch plates are shrinking when hot, or something in the lever, cable, lever, ball, push rod etc chain is shrinking?
My first instinct would be to check the adjuster on the face of the clutch pressure plate, to see if it is slack. Then check all the balls, rods etc from the sprocket side, before finally taking the left hand outer case off to expose the clutch and removing all the plates for examination; if nothing obvious has shown itself before.
I am afraid I would be at a bit of a loss if that didn't show up something amiss.

I hope that you find the culprit without having to go too far with the disemboweling.
Good health, Bill