I have stopped using Optimax and reverted back to normal unleaded. The engine and exhaust now sound quieter than the loud "BWOOAAAR" I used to get.
Incidently, I still have my Squadra chip in the car and the performance hasn't suffered to the extent that I notice any difference.
Oh yeah, the popping in my exhaust has almost dissapeared as well. (it dissapeared completely when I put the original chip back in the car for a couple of days).
So after all the good things I said about Optimax, I think it may only make a huge difference if your car isn't running properly. Most of the improvements to my cars running have been after I've changed bits like plugs and leads.
I know this has been discussed in another thread, but I can't hear any knocking or pinging using normal unleaded with my Squadra, but I hope I'm not doing any damage.