Hy.

I have done the Dutch nationals with the Mito 9C , thats the Mito with the centred carb. It does have other internals. Not slower than the Virtus or the Mito long stroke what i experienced on track. Maybe better end speed but could be better setup on the car though.
Absultely love this engine. Its fast, holds its tune, does not heat up in 30 min finals. far better than my tuned Picco EWC i had. Oh also very stable idle during fuel stops. Not exactly what you asked but hope it helps a little.
Does have regular size bearings wich one can buy cheap. So its if one has the extra room to accommodate for the offset carb, If not its the Virtus your best bet.