If Geno/CD can change Dolson I am pretty sure they can change Butler to be the best center soon or later.
A couple of thoughts here -
I really don't think that Butler is particularly heavy - some folks just aren't quick, folks, and exercise, etc., won't change that a whole lot. Don't know that Nat falls into the "slow" category, but, you can't teach "quick". You can teach recognition, judgement, but not neuron speed. Plus, some folks just have a big frame - no matter what you do, they'll never be built to do certain things as well as some other kids.
Coaches can encourage, lead, direct - but, they don't have a magic wand. Geno/CD didn't change Dolson: Dolson changed Dolson. Geno/CD may have provided the plan & support, but, if Stef didn't want to do it, and put the work in, then, nothing happens. It depends more on the player than the coach & staff.
I think Nat will improve because she wants to. Think about what she's gone through, giving up being a "big frog in a decent-sized pond" to come to UConn. She's fought through that transition, and then dealt with a frustrating injury. She's got the guts and the drive. She needs patience, and we need to extend the same to her.