Do all dimmers reduce max brightness of LED lamps?
Yes, some dimmers have a bypass at the top.  And the bottom. 

The game is really about wave-shaping, and nothing prevents a dimmer from giving 100% top voltage via a switch, or 99% top voltage via silicon.  However in practice, the wave-shaping is done to make the dimmer as cheap as possible while not hopelessly incompatible with LED.   Peak performance is not a priority.  

Speaking of "cheap", CostCo and Feit Electric are not "spending good money"... unless we're talking "throwing good money after bad".  Big-box is a sea of junk, it's basically the Alibaba crud but with the safety hazards fixed. You have to buy much more carefully if you want performance. 

On the other hand, pro grade equipment is more accessible than ever.