What is the binding power of starches vs flour?
When it comes to food 'thickness' (viscosity) is frequently discussed in 'relative terms' but not 'measurable units'. As such I'm not sure your question can be answered in 'absolute' terms. 

Turning to Ratio where it discuss "Stocks and Sauces" 


A Roux is 3 parts flour to 2 parts fat and the 'thickening ratio' is 10 parts liquid to one part roux.
vs.
A Slurry 1 part cornstarch to 1 part water(by volume) and the 'thickening ratio' is 1 tablespoon slurry to 1 cup liquid.


Arrowroot and potato starch can be substituted, in equal volumes, to cornstarch but they have differing outcomes based on the application (stews vs pie filling) where the difference is more than just 'thickness' but the things like the dairy or acid content of the stuff to be thickened. 

In the absence of an objective standard for 'what is thickness' I don't believe you can get a fully satisfactory answer to this question.