I hear ya....I think you'll be walking a fine line at 250 hps...and may even see a slight decrease in overall plant performance. Will they grow....absolutely! Will you save on electricity....yep! Will you be happy with the plants....well, that is yet to be seen. I like
@Puddentane cob idea....or a plain ol' regular led will certainly save electricity...but I suspect you want to stick with hps because you only will have to buy a new bulb, so I get it.
It will be an experiment that I think would be worth putting the time and energy into, in my opinion.