We always say...

Whether you spend the big bucks for a large two band brush or a Plisson HMW or a basic pure badger or boar or horse brush in the end they all do the same job. It's a matter of luxury and feel.

However, if you had dissimilar brushes, that is by critter and grade but the size and fill were the same and given perfect hydration and the same quality soap do you think it would take longer to create equal lather quality as you go from brush to brush or could you ever get the same quality lather from each?

Positively not. I have two (Mühle and Simpson best badger) that will not create proper lather from soap, whereas a Thäter (super badger, and therefore theoretically less suitable for use with soaps because of its softness) will without any problems.

So, no, not all brushes are created equal in my book, and unless you lather with forgiving creams (or soaps) only, I would go for an upgrade.

No, I certainly don't think they're all created equal, but then again I've yet to come across a brush which just doesn't work. In my experience they all make good lather given more or less perseverance, the big difference is in how much of it is retained in the knot, which is where badger scores over boar. For that reason I've long since given up face lathering hard soap with a boar brush as a dead loss, whereas a badger works fine whether best or silvertip. Given a soft soap or cream and a bowl though, the brush doesn't seem to make much difference.