@Grave:- A few observations......
Firstly for some strange reason when I try and take Screenshots running HiTM, I just get a black picture, so strange whatever, but I can't provide any to illustrate my points.
1) Stonehenge. I feel the free specialist to be just too strong so early on. If you manage to combine Henge with the Sphinx, thats a free scientist in every city from ridiculously early on, which lasts all the way through to Astronomy. Combine that with a beeline to code of laws, and then whacking out the Senate and going Uni Suffer, is 2 free scientists (or whatever) per city. You've probably already won if you manage this....
Last game (huge/emp/agg ais/rand pers/ me random civ/leader)...I got Germany and The Austrian Painter

....I manage to conquer the Greeks and The Nat Ams, plus achieving the above wonders by 500BC. I then filled the continent with eventually 32 cities, and found 3 religions. Ok so then I much later explore the world, and adopt merc, which leads me to point 2...
2) Mercantilism...you have it set at 3

per specialist...well on a huge map with astronomy and open borders with 9 of the surviving 13 nations, I discover Free Market, and change to it from running Merc
And my income (at 70% science, 30% gold) goes from +252

p/t, to -125

p/t.
This is where I took a load of screenshots to illustrate but as said earlier the didn't come out...so 2 examples were Athens, and The Nat Am ex capital (whatever its called)....Both were super food rich, and were running 8 specialists each (2 free, one from Uni Suff, and the other from The St Of Lib, built in 1350AD)....now thats 24

each. Free Market could never ever make up for that...even tiny cities were pulling in 6

per turn,(from specialists) which they could never hope to emulate from trade routes.
So in short, even though this was a slightly extreme example, even if I had open borders with every nation on earth, and Custon Houses and the like, Free Market could never match Merc. Plus Merc gets 2

per mine and plantation.
So a suggestion to Merc would be maybe 2

per specialist and 1

per mine and plantation, otherwise, the whole world just stays in Merc all game...