Some more thinking along the lines of "quotient with the one GPfarm case, but now with a more precise formula for the number of GPP needed to obtain n GPs has led me to change my values of different scenarios. The formula I used for F(n), the total number of GPP needed to get n GPs is
F(n)=5 x^3/3 + 55 x^2/2 + 325 x/3 + 165/4 + G( (n/10)-[n/10]),
where the G-part is a 10-periodic error term, not exceeding 81 1/4 (to be precise the errors are (41 1/4, 78 3/4,81 1/4, 58 3/4, 21 1/4, -21 1/4,-58 3/4, -81 1/4, -78 3/4, -41 1/4), the constant in the asymptotics obviously chosen to make this list symmetric under multiplying by minus one and reversing the order.
Subsequently I estimated the number of GPs acquired up til a certain point in a situation with k GPfarms as f^(-1)( GPP/ turn * #turns) - # GP farms/2 (where the last correction is added to simulate that you always have some remaining GPP in your cities, on average to the amount of half a GP per city.
Considering the relevant length of the game to be the area in which you have between 1,000 and 10,000 base GPP in your GPP farm (the lower bound also avoids some problems with singularities obtained in the calculations) and just taking as the value the combination of the maximum and minimum amount of the quotient of number of GPs obtained in a scenario and the base scenario (of one GPfarm for a normal leader), we get the following scores (1 GP farm, 2 GP farms, ... , 5 GP farms in each scenario). Note that in most cases the maximum value is attained at 1000, and the minimum at 10,000.
nonPHI, non PAC: 100, 116-117, 128-129, 138-139, 146-147
PHI or PAC:119-126, 144-157, 162-178, 177-194, 189-206
PHI and PAC: 134-148, 165-187, 187-214, 205-235, 225-259
Note that the values are all considerably higher (increasing the value of extra GP farms and the PHI trait). Also there are quite big changes in time of these values, but the general idea is clear.
Also the decrease in time suggests that the relative benefits are highest in the earlier game, which corresponds to the cubic term in F(n) taking over the quadratic term only after 16 GPs (which is the order of size we should consider changing the growth in number of GPP necessary for n GPs to become cubic instead of quadratic).