I edited my above comment to include the wattage of my MH ballast, forgot to include it when i wrote it! 

If your lamps running good on 100w MH, then It sounds like a good match after all, glad to hear it. Members on here appear to have success with both 100 and 70w ballasts with mercury vapour lamps

The "100W MH" ballast would power the MV lamp by almost 120W. The lamp may stand it, it may be even more efficient, but it will degrade faster. However there were many cases of 250W MV's operated (accidentally?) on 400W ballasts for years (till the scheduled group relamping), so the chances the lamp will handle that overload well are rather high.