I replaced mine with MB ones 2 months ago and experienced slower response and lean mixtures. When I replaced them with the original ones, the performance improved a lot. So much so that I had to adjust my mixture settings as it was too rich. Wonder if the new diaphargms had anything to do with the mixtures?
The diaphram picks up the slide and the needle (in response to intake vacuum) which changes the size of the fuel orifice because the needle is tapered. For the same amount of intake vacuum, a stiff and heavy diaphram will not move as far as a thinner, lighter one.
The trick (for me) is to find an affordable substitute for the stock diaphram that will not change the carb's behaviour too much.
Some people also replace the stock spring in the carb for the stromberg. If you do that, you also have to move the needle setting to the lowest groove because the spring is beefier.
Everytime to change things in a carb you are going to get performance changes. The %#*^ things are rather sensitive.