In a recent 5.3 build done by Camaro Performers (sept 09) they used the factory 4.8 flat top pistons (which have a moly coating on the skirts too
)
to bump the comp ratio up a little. This engine is going in "Track Rat". With a 58cc chamber(Trrick Flow heads) they got 11.5:1 compression. Yours would be a bit less. And they are still running 91 octane gas with no detonation according to the article.
If you can't find the article PM me and I will give you a full run down on it.
As for your original question, I have no idea if it is safe to deck that block. But a set of factory pistons seems like a good option.