Espresso - Dose affecting extraction time disproportionately I have a Pasquini Livia 90 and a Baratza Vario Ceramic grinder. 

I'm finding that the only way to tame my extraction speed is by over dosing the basket. Specifically, by loading the basket so heavily that it pushes up against the shower screen. 

If I use a double shot basket, I have to load 21+ grams to get the extraction down to a brew ratio of 2:3. Basically, I have to get it to the point where I'm struggling to insert the portafilter, otherwise the extraction speed is just incredibly fast. At the recommended starting point for the Vario grinder for espresso, (2Q), it takes about 7 seconds for an extraction using 16 grams of coffee in a double basket.

If I stick to the recommended doses, and give the coffee some headroom from the shower screen, I end up having to use the finest possible setting (1A - 1E) on my grinder to 'slow the flow'. This results in extremely overextracted tasting coffee, which makes sense because the grind is like powdered sugar at that point.

What am I missing here?
I would not exceed 20 grams on a twenty gram basket. Weird things start to happen. 

I would make your grind finer - and simply stop your shot quicker. If it tastes overextracted, then stop your shot even quicker the next time. Adjust accordingly until you get the sweetest shot you possibly can. 

Your general espresso recipe for a well roasted coffee should be somewhere around:

20 grams of coffee in
extracted for 26 seconds
A yield of around 30 grams of espresso

I hope this helps and good luck!