Reusing stretched timing set....?

This topic is getting a bit long and loose too... :)

Put a degree wheel on the crank with the old chain attached. Turn crank back and forth and measure degrees of slack.
I recall those 4 degrees on the sprocket are usually crank-degrees. Which means the cam only sees 2 degrees change.
And yes you can sure feel the difference, but it depends on your personal inbuilt ability to sense technical improvements, or not.

Even though I never want to see any play in a timing chain, I would run it, especially with the chain-tensioner.

Even if the old chainset has 2 degrees of retard, I would install it in the 4deg advance way and have 2 degrees to 'wear' in the future as spare :)
A too far advanced cam will build more cylinder pressure (and torque) but might run out of steam above 3-4k rpm.
Beware of pinging though.