You must run premium gas from now on. The stock normally aspirated pistons require 91 in the US. If you turbo the stock setup you need to run premium.
You'll still probably ping if you go above 5-6psi with the setup you are talking.
The mechanic is totally wrong about the timing. You need to retard the timing
under boost conditions to avoid pinging. The stock 240T distributor has advance
when the engine makes vacuum. When the engine makes boost the mechanism on the
distributor retards the timing. To run a setup you are talking about you must retard the timing. Retarding the timing and water injection will help but your setup really can't run high boost with so much static compression. Also a lower compression ratio makes more room for more combustion. This is why any high
boost pressure motor runs low static compression. A good idea but it does require careful setup.

Another thing you can do is to install an intercooler more efficent than the stock volvo one. You'll have to have this made for you.

Dave 82 242ti 278k mi.