As a blm who stacks SSP (currently at 643 with food) i can testify that Selene buff gives me the same boost than foe's requiem or even slightly better depending on the situation.

This has been tested a lot by myself in fights up to T11

ofc im using B3 T1 B during my mono target UI phase to max out the SSP benefit (Puros spreadsheet explains how very well)

Imo this is prolly the best buff or at least the one i prefer in terms of gameplay / feeling. Please use it any time you feel comfortable to do so