Hi Tiennetti,
I've seen the bit in the FCOM I think you're referring to:
For crosswind takeoffs, routine use of into wind aileron is not recommended. In strong crosswind conditions, some lateral control may be used, but care should be taken to avoid using large deflections, resulting in excessive spoiler deployment which increases the tendency to turn into wind, reduces lift and increases drag. Spoiler deflection starts to become significant with more than one third sidestick deflection. As the aircraft lifts off, any lateral control applied will result in a roll rate demand.
It seems to refer only to the roll rate demanded once airborne.
As you say, during the initial rotation the aircraft will still be in ground mode, so I suppose the only way that aileron deflection could result in an increase in rotation rate would be if greater elevator deflection were commanded from the same amount of backstick if there was a simultaneous roll input...I have never read anything in the FCOMs or anywhere else to suggest that this is the case.
I'm starting to seriously doubt whether what I was told was true (or perhaps I just imagined it...)