I think it's worth pursuing. Your VE just doesn't look like it's performing properly, to me.I'm considering returning it to have it checked again.. I'll run a few more shots at the wider focal lengths and see how it performs,but the primes remain my first choice.
It could be sample variation ... Zooms are more complex than primes and more likely to have tolerance issues or decentered elements and yours does seem to perform better than mine or Stuarts.
Rob
Bon chance!!