Flow rates in the ECU for 200BHP injectors are lower (this does tie up with bosch injector tests)
You can see actual values here - injector opening time based on rail pressure and injection quantity
Basically what it means - to deliver 80mm^3 @ 1600bar 200BHP injector has to open for 935us
Same delivery with 210BHP injectors will only take 902us.
Because map for 210BHP injector does not have this exact point, it has to be interpolated based on points around it.
Now, for example using 40mm^3 @1600bar as it's common for both maps
210BHP - 596us, 200BHP - 615us
I can get about 15% increase from standard 210BHP injector by just modifying shimming inside the injector (cheap) without causing any adverse side effects, or 30% but it will also make pilot injection go beyond where it can be adjusted back (not tested yet - it might actually be adjustable), similar modifications to 200BHP injectors did not yield results so good.
For not much more than just messing with injector calibration we can stick a higher flowing nozzle in it.
So target of around 30% increase is achievable - this should make it possible to get a flat 600Nm down from 2000RPM-ish (air limited) up to 4500RPM (or even up to 5000).