That’s an interesting question and I’m not absolutely sure of the answer….. Testing some of the very early Joe Meek units recently, I have not noticed any change in performance of the cells… they certainly don’t wear out like tubes (valves). I think it’s likely that the life span is very long.
Regarding the technology; I used a later and faster type of opto cell in the TFPRO gear, it gives a greater range to the attack and release, but the performance of the P2 is very similar to the earlier VCTwin. The P2 was a sort of ‘crossover’ product’, there were changes to the circuitry but the performance was similar. Now we have the P10 which is completely different with its increased facilities and very sophisticated compressors.