Sorry for rushing, but paying customers you know....
So I drew a quick what you get with your two methods of settings: I did not even bother with the other person's settings as they would simply overload the hitachi and cause it to shut down on overcurrent with 50hz/240v base speed settings as you know.
Sorry for the paper I used: we have put on servo/spindle training classes for the R&D engineering department at corporate P&G and I stole a bunch of cool paper for stuff like this...
So your first way was to sete base as 110v & 50hz. This is the dotted line of course in the pix. So from 0-1500rpm you get all the motor is rated for, but above 1500 you loose everything! you are stuck 110v@ all higher speeds so your full rated output goes down from 2.2kw to 0.97kw max! look at all the area above the curve you lost.
Your second way was to set base at 266hz/8000rpm: again, look at all the HP you throw away! it is much whimpier than method 1 from 0-1500rpm, but does gain at higher speeds.
Remember the PURPOSE of the chinese POS hi speed spindles: MACHINE AT 18000RPM! So they don't CARE that you loose all that capability from 0 to 10,000rpm....
What YOU want is to set base at 110hz & 240v and GET IT ALL. NOw you can machine steel and stuff from 1500-5000rpm and not have whimpy-ville.
this is 50hz*240v/110v. same v/hz curve but don't give away the store.
You mentioned wimpy at 50 rpm: SHOULD NOT BE - i suspect that was with the 8000/266hz base settings. We sell Hitachi. It should be able to produce full rated torque down to like 2 hz on sensorless vector....
Adding an encoder will just get the 0-2 hz full quick torque back - and position if you need tool changer or something. Otherwise it should not make your performance any better.
I will not go into how from about 5000 to 8000rpm you loose potential torque due to breakdown torque area of the motor - more complex. Not appropriate for this discussion. Just realize that when we design high speed spindle motors, we MUST use reduced voltage at "base speed" in order to not get bit by the V^2 loss of torque at the high end.
'snuff for now - gotta get back to it. It is again miller time and I get to play with my 40 mile range 16db M900 rocket 900mhz links to get cable to my house thru 1 mile of solid trees back in the woods.... the important stuff!!! Hope you enjoy this.