TL;DR
An agricultural towers quality could affect the speed at which trees grow.What
Add growth speed bonuses to trees in range of quality agricultural towers.I don't think the growth speed should scale similar to smelters/assemblers, but just give a few percentages on the lower qualities. Not more than 50%, probably more around 25% at legendary quality, so the agricultural tower does not look to ridiculous.
Why
In most other machines in the game, the usefulness of quality often means you save on space. 5 normal smelters can be replaced with 2 legendaries for the same amount of processing, primarily saving space.Allowing trees to grow faster with quality agricultural towers sort of mirrors that to a degree.
Effectively you will need less space (and towers) to reach the same output of fruit.
The in-lore explanation would be that grooming, nurturing, pruning, internal composting etc. are better which allows the tree to grow more effectively and efficiently.