Mutt
I have to disagree.
If you have a runway where by 140 kts you have reached a high enough speed to lose one engine then if you derate you will take longer to get to 140 kts and take longer afterwards. Most likely V1 (one engine out)will move towards Vr ie you need to get as close as possible to lift off to stand a chance of making the obstacle clearance.
In the meantime V1 (last chance to stop) reduces because, for any given speed, you are now further down the runway.
The very least this will do is to narrow the range of V1s.
My point is you are always able to narrow this band to zero by choosing a suitable de-rate. When you have done this you have - in effect - a balanced field ie you are seeing the same balanced field that would be seen by another aircraft fitted with less powerful engines - less powerful by the factor of the chosen de-rate.
In practice you may not want to de-rate to this figure because you would need to put power on after lift off in order to get a decent climb out, thus more-or-less defeating the point of the exercise (being kind to your RB211s). Maybe this is why you don't recognise the link between de-rates and affects on V1 range.