From my experience with stock burrs (steel) the stock retention is about 3 grams as you mentioned. Problem is it tends to be quite random, so it's a bit frustrating in single dose mode. Generally you will add a few grams more than you need and either you get the right dose or just remove the extra.
Also bought the blow up hopper. The design is a bit flawed from the factory:
[*] Blow up hopper still has vanes for the chute
[*] It does not fit flush in the grinder: it has some creases which actually grip the funnel.
[*] The lid doesn't have some sort of valve, so on the upstroke it will pull up air and grinds + chaff from the grinding chamber. All this will then come out of the vanes and not tight fit and on the outside of the grinder + motor vents
It's not actually visible but over the span of 1 month for example you get quite a big deposit of coffee on top of the grinder ( also under the little plastic cover on top and probably on the motor inside)
Luckily you can solve this problem by taping the neck and the opening for the hopper chute, thus creating a tight seal.
I am now getting between 0 and 0.2g retention with light and medium coffees. Also the grinding is quite fast (4s ) and very little popcorning.
What I do is I also keep constant pumping during grinding and use a portafilter funnel.
Another thing to keep in mind is that darker roasted coffees might not work so well in this setup. Lots of static means the pump won't be able to clear the chute.