Even with upgrades, the drawer will show Stack Limit: 64 (x8) in the WAILA tooltip.

As far as I can tell, the WAILA label is doing the right thing and taking the upgrades properly into account.

You did skirt into a bigger bug though, which is common slot calculation was not taking the configured base multiplier into account (which is 4 stacks, by default).

Check 6.0.1 and if the rendering looks wrong, please follow up with some specific examples/configurations.

Ah, my mistake - I was looking at what I thought was a non-upgraded drawer and assumed the (x8) meant stack size with a limit of 8. I did a double-check and indeed, a non-upgraded drawer shows (x1) which I now know is the storage multiplier.

I'm glad I at least pointed you in the direction of an actual bug, though. :)