Now that it's been live in game for some time, generating real-use performance metrics etc, can some serious consideration be put into bumping the limit up massively?

At a very minimum I'd say to 1000, if you want to err on the side of caution performance wise, but in that scenario you should still re-visit and increase again if the performance metrics support.

I can't help but get the impression that when devs came up with the 500 number, it was via something along the lines of "how many slots do we need to give to allow up to the 95th percentile of players to do away with bank houses". This is a terrible metric to base a choice of limit on, because the ones complaining the most are the ones in that last 5th percentile.

When the vault was released i went around all my bank houses and placed only the largest stacks in the vault. Vault was filled with only items where the stack size was 10+. After completing that process (it took about 3 days!) i'm still left with around 30 bank houses to cover the smaller item stacks.

One of the main benefits of the vault is supposed to be browsing and placing items directly. But I already have a good idea what most of the stuff in the vault looks like (because they are common items), so the benefit to browsing is minimal, it's the rarer items (smaller stack counts) that are beneficial to browse when looking for a furnishing but I can't do that as storing in the vault would be inefficient.

I'd like to be able to give you a realistic number by visiting each bank house and counting the unique stacks (after retrieving to inventory) but after 3 days to get stuff into the vault in the first place i just don't have any more energy to spend on it for now.

note: don't assume that anyone not at precisely 500 stacks in vault doesn't need this, I'm a little under just to allow space for new stacks of 10+. Probably anyone at ~450+ would benefit from this.