Centaurus...on myboeingfleet site there is a chapter called something like 'performance engineering training'..
There it says ,that you should use actual weight take-off speeds,even if you do reduce.The same that you said.
The V1 will be much lower,sometimes more than 10 kts,for low weights when you do a reduce take-off.
I had the same argument with an engineer,and I had to ask around.
The engineer told me that the inertia will be smaller with lower weight,but I also told him that the braking force will reduce with lower mass.....so 1 to 1
so,I take boeing word in this case:use actual weight speeds.It seems the normal thing to do.Use REDUCED power,with higher speeds (for higher than actual weights) ,this doesn't seem right.
maybe for some engineer,but let him answer in case of eventual overrun...with 20 kts higher V1.