my two pennies worth.
i've got 13" 4 pot porsche boxster S brakes up front and a disc conversion on the back.
when i first installed this i drove it without the servo connected and was surprised how much effort i still needed to stop the bus quickly. now i'm not saying that it was the same as the stock brakes, but it still took a fair amount of pedal force.
however, when i connected the servo up, this is when i benefited from having the discs.
personally, if i had the opportunity I would have liked to see how my original brakes would have handled with servo fitted. again, my bus is a crossover so i had discs as standard. but the mot man did tell me that my stock brakes were the best he had experienced on such an old vehicle
Nathan