As long as your firmware actually says it's 3.55 you're good. It's really not all that different than modding PSPs, you just have to be a little more careful. If you still want to attempt it and need help with anything, feel free to PM me.
As for this PS2 Classics hack, I've been messing around with it for a day now and several games are working without any issues (only game I tested with a very minor sound glitch was FFXII International). A few other games have been appearing that have more issues or won't play at all, so the games on PSN are more or less modified in some way that eliminates the few bugs or glitches that would exist otherwise.
There's even a compatibility list up.
http://www.pshomebrew.net/wiki/PS2_Classics_Emulator_Compatibility_List