I had a '50 plymouth and did alot of research on this. Luckily those mopar flatheads had hardened valve seats so were OK on unleaded.
The deal with lead is it acts to cool the valves & seats so they dont get super heated and soften up, which then allows them to burn out.
I've always heard its just a matter of time and how hard you drive it. Non-hardened seats will eventually burn out. If you really romp on it, you could burn them out in a weekend. Putt around and maybe they'll last years.
Additives help some. Not as good as the old "real" lead additives but certainly better than nothing.
Also, with respect to compression ratios - yes high CR will burn them out faster, but all Model As running unleaded will burn valves like crazy and they have a CR of like 4:1! Stainless valves and hardened seats are a must upgrade to run unleaded unless you are idling in parades.