There we go again with the useless speculation.
SPU2 plugin is based on the PEOpS Spu plugin, obviously. To make things short, we sent our sources to Pete asking him for advice, ofcourse, we had the spu2 fully documented and all the registers were there.
We arent coding ps2sp to be the '!33t' or the best plugin, we are simply doing it for fun and to give something back to the emulation community. It will be released when we feel its ready
PS: Yes, ps2sp sources are closed, only a few people "namely pcsx2 devs/testers and Pete" saw the source. It shall remain closed source till the project is stopped, which aint happening any soon, hopefully
Yours,
-Elly