Agree that there are some quality brines out there, AGY might be...

Agree that there are some quality brines out there, AGY might be one and Jaderite based ones as well, but those are not as pronounced as you are suggesting and most brines including those in the Atacama desert have Li20 ratios ranging from 0.02%Li20 to 0.32% Li20 (see link below), much less than hard rock plays in general.
http://www.saltworkconsultants.com/blog/lithium-in-saline-settings-1

Overproduction doesn't benefit too many companies as a general rule. If over-produce, sales to third parties arise and then they drive down your product prices because of the need to maintain project economics. Obviously if SQM can keep all their over production for themselves that will aid price stabilisation in Ag end markets but I dare say that brine producers will sell the key inputs (potassium chloride/potsash base if over produce) of the production process for cash flow issues rather than have an inventory build up beyond what is commercially acceptable. Just a view, but agree that each participant will need to assess against their own needs and for hard rock plays utimately they need to stand up on their own financial basis etc etc.

All IMO