I use the Paiste cleaner on the newer cymbals. It's very good. Finish it with the clear coat too. I cleaned an old cymbal once to new condition and recorded it before and after. There was no discernible difference to me. I think a cymbal would have to be green to make a difference but everyone hears differently I suppose.
Yes, I think once the oxidation really kicks in and the cymbal has a green hue, sonically it will change. What made me want to really oxidize my New Beats was having played a friends set that had been really oxidized. The sound was a bit softer than my set.