We are getting Santos Mahogany stair treads in our boxed stair case. The risers would be painted white. Are paint grade MDF risers alright for the purpose? I hope it would not be a weak link. Any expert comments...?

MDF will work fine for your painted risers. The only problem with it is if you have outside mitered corners done in MDF, they are more prone to being damaged. In that scenario, I recommend a hardwood like poplar or maple. Both paint well.