You can expect a 2.1% speed increase per level of magnetic field weakening.
0 is nothing and 9 (max value) is 20%.

Compared to around 16% increase from Leaperkim "high speed modes" which also use field weakening.

While calculations i have done on the ET Max show +28% speed increase with field weakening. I still have some more data to collect though. The GT Pro free spins even faster!

00:00 FW Update
05:40 Lynx
07:49 Cruisin'
19:50 Inmotion V14
20:30 Burn