Realistically you are supposed to be using Hasty Hand on stuff you're already really over ranked for, atleast to have good chances of success. Honestly the difference between 20 and 10 SP isn't a big deal. What matters is I didn't have to spend a bunch of time crafting that 20 SP.
This skill is mostly for parts - which gives you small amount of SP anyway.
As for this, it's totally untrue. The determining factor for SP gains is Your Rank VS. Optimal Rank to Craft. Parts, Materials, Finished goods are irrelevant for determining SP gain. The only differentiation in Your Rank VS Optimal Rank to craft appears to be in leves, which seem to apply a 50ish% bonus to the SP gains (It may be more like 25% but just for arguments sake). Basically Hasty Hand if you want to spam up a bunch of something quickly (Arrows, Spetches, Buckles, Lumber) typically that you are ten levels over. If you are worried about the 10 SP per synth spam standard 5-6 times your choice.