The annular boostered (8-hole version?) should've worked just fine. Remember that the annular boostered carb isn't the same jetting as the drop-leg version due to the increase in signal.
Now in comparison of hole shot, the smaller venturied carb will most definitely respond quicker.
Did you play with jetting and float height? Squirter size? Etc. That 850 could have a 31 or higher squirter where as the 715 might have a small 25 or 28.
Does it fall on it's face out of the hole? Stumble? Etc...
Don't give up so soon! wink