I realise that every amp is going to vary, but is there a ball park figure to how many watts a 50w plexi will put out when dimed?

I just got a couple of Eminence Wizards, and if one isn't enough on it's own, I'll convert my combo to 2x12.

Unless its a 1959 in which case reckon on it putting out 200watts + when fully cranked.

You'll probably want two.

I would expect one could handle the power.  IIRC, Eminence rates their power handling in continuous numbers, so they can handle peaking above the rating.

BUT...Eminence is outspoken that they do not recommend using Wizards in open-back cabinets (it's even covered in their faqs), and I expect your combo is open back.  The issue is cone cry from the lightweight cone and super-efficient motor, you probably won't want to push a Wizard too hard in a combo and have an additional speaker to help carry the load.