A former colleague was similarly concerned about his 1960s Mercedes 280 SL designed for high octane leaded gas. He wrote to Shell Oil and asked about lead/valve seat recession etc and received quite a technical package back which he let me read some years ago.
In essence, provided the octane is high enough to meet the compression needs of the engine, and provided the engine is not 'loaded' heavily on a regular basis (racing/towing etc), the degree/pace of valve seat recession should not be an issue to concern a collector car owner; however, hardened valve seats still represent the safest and longest lasting solution.
