I've run fair trials with reasonable numbers trying to see if I could push the bees a bit with subs to offer a better service to a commercial orchard I pollinate.
Results after a number of years and hundreds of hives indicated the sub (ultra bee, bee pro and a homemade mix with soya flour and brewers yeast) were all irrelevant, the hives that had slap (sub only) were roughly equivalent to the controls with nothing, whereas the hives that got just a slurp(syrup feed) or slap and slurp developed a bit quicker than controls.
My conclusion is that my bees have enough pollen but not enough nectar so supplementing the nectar is of far more value to them for early development.
Autumn trials didn't produce bigger colonies in spring either.