In the absence of first-party data from amazon, it was guesswork a few months ago. Sellers were dependent on third-party data. 

Amazon has solved this by providing the search query performance report. 

SQP report includes impressions, clicks, add to cart & purchases. It includes the brand share of all the above metrics. 

How to know the available headroom?

There are a couple of ways to know this. 

Download the brand view report for a quarter. Filter the product category search terms with purchases that you want to compare. Divide them into brand, non-brand & generic search terms. 

Plot category purchases & your total purchases on the graph. This will enable you to analyze available headroom.

Similarly analyze the headroom for brand, competition & generic search terms. It will give you a clear indication of where to invest the money for maximum returns. 

If you want to save a few hours, switch to a data analysis & visualization platform like Hector. 

Hector will have this data ready for you. You can analyze this at a brand & category level.

Curious to know more about Hector? Click on the link in the first comment.