One question though, does the willow quality matter if you are only using it for tennis ball ? The ball is anyways going to bounce if its EW or KW .
let me put it this way, i have tried inexpensive KW bats (GN Evo slayer) and high end bats (T-rapta Fury 100, L&W Reserve) in tennis ball matches and found the following:
i) the ball flies off the bat irrespective of the willow -
if you middle it ii) if you don't middle the ball, a grounded shot is better with a cheaper quality willow as the ball will most probably not carry all the way due to:
a) There is not as much wood in a lower quality bat as a more expensive bat (for the same bat weight)
b) bat speed / pickup is not optimal for a lower quality bat by virtue of the bat design and you will get the best momentum with your swing. For example, a 2.14 KW from a random vendor and a 2.14 EW designed by L&W, will pickup very differently.
Also, since the tennis ball bounces much more unpredictably than a leather ball, you really want a light weight bat that has a lot of wood to ensure your shots are traveling the distance. if you can find a KW bat that has good balance with a light pickup, sure - go for it!