I'm trying to understand what you wrote as it is not clear to me.
Are you suggesting that the head metal is not a sufficient heat sink to keep any exhaust leak from unevenly heating the head components during warm-up?
Not at all. It's just that the exhaust gasses flow across the exhaust valves..and stems on their way out and they get very hot of course. If cold air is pulled-in against them, that sidemaking contact with the cold air will contract, warping the stem and depending on the amount of warpage, can cause the stem to bind in the valve guide when closing. If it is not fully closed during one stroke, there is the possibility of the piston hitting the stuck valve(s)...and that's the end of the engine...or at least the top end. During overlap, that's when the piston is at TDC of the exhaust stroke, both intake and exhaust valves are slightly open at the same time. This is for proper scavenging but for a split second there is a sight flow-back from the exhaust system. If that flow happens to have a stream of cold..as in just ambulant air.. with it, it can..not always.. effect those valve stems depending on how hot they are.
In short it's just a good idea to fix exhaust leaks..just to be safe. The other thing I wanted to say is that when I was taking pressure readings on mine before doing my exhaust mod...and this was about the time when I too have some spring joint problems, with all sealed and the spark arrester installed I was able to get a ridiculous 4.2 psi reading at 6200RPM and at WOT. I expected ounces..even something under a pound but not over 4psi. No wonder those spring joints get pushed apart! With the arrester out that was cut to 2.4psi but it was so lean it didn't run right in normal operations. I added an HMF arrester and it was then borderline but did loose some low end. Now that I have the OE arrester back in and adapted to the new end I did loose some top end power but gained back all the lost lower end so...that cool with me...and it's a little quieter.