Did more testing:
I increased idcmax to 120A, and it gave a boost in performance from about 15km/h to 40km/h. Should be more than enough acceleration in stop-and-go city traffic. Seems to peak at about idc=80 to 90A which is somewhere around 30kW.
Then I tested with syncadv=40 and syncadv=35. Couldn't really tell a difference to syncadv=30.
Then I took off with syncadv=20 and after 40km/h decided it was so slow it wasn't worth testing. Conveniently I had left the capturing off also, so no graph of that one. I stopped, switched to syncadv=50 and continued the run. There wasn't a considerable difference to syncadv=30.
For the way back I switched to syncadv=60. This made the inverter's reported values go way off, showing like idc=10A at 80km/h and negative idc when in fact there was no regen, more like automatic throttle. So clearly there's a reasonable limit to this and syncadv=60 is beyond the limit. This actually makes some sense: 300Hz * 60dig/Hz / 65536dig * 360° = 99°, which means there's more than 90° of phase offset at 300Hz, in addition to the 45° caused by the id=-iq configuration I am using, not mentioning what the FW controller is adding to id.
Interestingly, when calculating the same thing using syncadv=30, at 300Hz basically all the current is id and iq is zero, to which you still need to add the additional id added by the FW controller. As a result the inverter output phase is more than 90° advanced to the resolver position. Of course the inverter doesn't show it as such as it doesn't realize what syncadv is doing to the actual phase offset.
Then I reverted to syncadv=30 and tried a throtiq=1.8 throtid=-2.2 configuration. The differences were too small to properly notice, but it might have resulted in less oscillation when driving slowly, but worse high speed acceleration. For the way back I swapped the parameters to throtiq=2.2 throtid=-1.8 and the result in comparison was the opposite, more slow speed (15-50km/h) oscillation and better high speed acceleration. In comparison to iq=2.0, id=-2.0, it wasn't better. Keep also in mind the battery voltage had dropped about 9% already, which also worsens performance.
I don't know what to make of this. I think what's interesting is how wide the well performing syncadv range is (maybe 25...50), yet how far it is from the recommended value. What also is noticeable is I can't seem to get the motor to accept more than about 15kW at 80km/h (300Hz).
EDIT: I think I might have to start doing some tests with the FW controller disabled.
EDIT: Graph of the idc=120A test