you can purchase great sceintists with faith thinking it may be a good long term strat but noticed the cost of them goes up fast.
had all possible scientist specialist slots filled at all times (constantly generating great GS points) - the main purpose was of course maximum beaker output ... Purchasing great persons with faith kind of interferes with the natural generation of GS (point costs increase ..) ... The religion that invaded my cities allowed me to spend my faith on cathedrals ... And a GS very late game ....
EDIT - no matter the victory condition type (scientific, diplomatic or domination ) - you must be able to keep up in beaker per turn output ...