The energy savings from using the higher voltage come from the ballasts running slightly more efficiently. A ballast consumes about 10% of the rated power, on top of the rating (the rating is for the bulb, not how much power the ballast draws) so a 1000w ballast would use 1100w or so (it depends how good a ballast it is, some are more efficient than others) but will use slightly less at 240v. The majority of the power consumed is going to the bulb, and that doesn't change, so any savings are marginal and not worth investing in expensive equipment over.