ping on topend

What kind of fuel are you running?

Yes, it's very harmful for the engine to experience that. Especially true when you're boosting it or using nitrous. If it was minor as you described, and not for a long period and you saw no ill effects, then you're probably alright.

I honestly wouldn't want to drive the car if he squeezed that little of power out of it and it's pinging. If you don't have an alternative mode of transportation, then I'd highly suggest staying out of boost until you can get it re-tuned.

Fuel is obviously an important issue, but it seems like you have a setup that would support 10lbs of boost as long as you're running 93 octane, or 91 octane if that's all you can get (as long as it's tuned accordingly). Unless you are having a fuel delivery issue, it's probably a matter of too much wide open throttle timing.

You could pull your spark plugs and see how they look. If they are beat up (white (running lean), closed gap, excess deposits, physically damaged, melted, etc.) then you may be looking at something more serious)

Definitely stay out of boost until you get the detonation issue sorted.

The car needs a tune by a person experience with stock-internal blown 2Vs. This generally means being conservative on the fuel (i.e. fat) and timing (i.e. retarded). The tune should really be performed on a load-bearing dyno with a wideband AFR -- checking the exhaust stream before any catalytic converters -- and logging to verify timing across the RPM and load range. It should also take into account the fuel you'll be running (e.g. 91 octane?)

Your fueling combo sounds solid but you may want to get a fuel pressure gauge on the rail to verify no issues there.