It's very unlikely that the OEM is accounting for any change in aerodynamic drag on the runway due to a change in stab trim setting; to be brutally honest, there's no way they could identify the effect in any typical test variation.
The possible mechanisms are:
1. Reduced trim drag will improve your performance and give you a higher max allowable TOW if you were WAT limited . Won't do much for you otherwise.
2. Reduced stall speed (due to reduced trim downforce) will give you a lower minimum V2. That may reduce V1 also, depending on the balance of the various factors, and that may allow a higher TOW if you are field length constrained.
If you are light enough that there's no WAT concerns and you are VMC or VMU limited chances are you'll not see much if any difference (although VMU certainly does depend on cg, it would require a bunch of extra tests to take credit for moving the c.g. back if you were limited, and I very much doubt the OEM did that)