Stockfish 8 scalability (1 core testing)

Time control
Hash
Result (200 games)
Elo gain
10+0.1 vs 5+0.05
4 MB
82-12-106
127.0
20+0.2 vs 10+0.1
8 MB
76-11-113
117.2
40+0.4 vs 20+0.2
16 MB
70-8-122
111.4
80+0.8 vs 40+0.4
32 MB
57-10-133
83.2
160+1.6 vs 80+0.8
64 MB
56-11-133
79.5
320+3.2 vs 160+1.6
128 MB
41-5-154
63.2
640+6.4 vs 320+3.2
256 MB
31-6-163
43.7
1280+12.8 vs 640+6.4
512 MB
29-5-166
42.0

On these data I applied the least squares method for my theoretical model (ELO = A * ln(depth) + C),
and the model explained 99.87% of the variability of the measured data (see the image below).
The absolute value of the Elo is provisional.

Conclusions:
1) Somewhat lower Elo gain is expected under hash pressure;
2) Somewhat lower Elo gain is expected on multicore systems;
3) Scalability is almost perfect up to depth 20;
4) Some possible improvements are expected on higher depths.