Yeah I agree its pretty doable. Just won T258 my first try on Emperor (Immortal player in Vanilla)
Thats still 1700s I think. Could be optimized better than I did for sure. This was a Russia tall game so not even a strong science civ.
A couple of notes for those trying to go this route
1) GS (or other GPs) you purchase with faith do not affect your GPP threshold (Whereas "free" ones from wonders etc do). Micro accordingly, esp when you finish the hubble (Also buying say a GS with faith doesn't increase the cost of the other GPs just the next GS)
2) Research Labs are now (sensibly) on the science win tech path so prioritize them
3) As has been said before getting to 900-1000 bpt is very doable by T220ish or earlier which means a GS is worth 7000-8000

or pretty much any tech except the last level
4) Even my best RAs at the end with Rationalism and PT boosts were only netting about 4000

so my take is that they may help you shave off a few turns but unless you have like waves of 5+ (which is hard to do) they are kinda ancillary...
EDIT: Forgot to mention that one of the reasons it would be easy to improve on my time is that I can skip the revenge destruction of Siam who pissed me off by stealing too many techs (even though we had a DOF and I told them to stop - lol!)