GP farms tend to have crappy multipliers unless you're doing the whole specialist economy shtick. Don't settle there, if you're going to settle, settle them in oxford capitals.
And really, specialists are overrated on lower difficulties. A scientist eats 2 food and shits 3 science. This means you need 2 grassland farms to feed a single scientist! It means you need 3 population and 3 happiness to generate 3 commerce, with no food surplus! You know what beats this? Lighthouse coasts! Each citizen generates 2 commerce, and they are food neutral.
Ah, but what if you have the mids. You know what also matches rep scientists? Lighthouse coasts as well!
So no, the appeal of the specialist economy are not the raw commerce, because you're better off working tiles. If you're going that route, at most, you want to use your specialists to eat up spare food so that your cities don't grow into unhappiness, but it's not like those pop points are worthless when you have slavery. No, instead, the appeal of specialists is in the great people points. I think it was Unconquered Sun who made the point that every single GPP was the equivalent of 40-50 beakers. You then turn those beakers into more beakers with promiscuous tech trading. This really works on Immortal-Diety, where the AI have enough of a tech rate boost that it's the only way to keep up and stay in the tech race.
Of course, the way the GPP system works, if every single city runs specialists, alot of them would go to waste, so you want to centralise your GPP production to only a couple of cities max. Even then, that's very short-term, lightbulbing runs out of steam as GP costs ramp up and bulbable techs run out. So yeah, you want to turn your short term economic boosts into a timing attack that secures you alot more land with the specialist economy, instead of using specialists for raw research ability (cottages have that covered). If your breakout war fails, then you'll be so far behind that you'll lose.