You can easily test that by making two (or three) versions, each with lower MB. I never use more than 8-bit, usually tifs with an alpha channel for opacity around 2000px wide or high (stems/barks are often longitudinal, and may be only 500px wide). I like tifs because jpg's are less predictable when I lower opacity to 0.6 or so for more light in canopies. Unless you need plants for really close-up shots (and then you also need perfect plants), that's enough (for me).
