Sometimes Hodgdon, who owns Ramshot now, has load data they don't publish. I'd highly recommend calling them as Grand is such a new powder. Call Hodgdon or Ramshot and ask. Wouldn't surprise me at all if someone has loaded the 257 Wby with Grand and just hasn't published the results yet...
I haven't. Didn't have much Grand to work with, but I surely have looked at my factory fresh 257 Wby cases and thought about it...
That being said, I nosed around a bit and there seems to be some chatter about Grand powder in the 257 wby but it appears it's only like what you are seeking. Load data and people asking about it.
It is right beside Ramshot Magnum on the burn chart....which is kind of odd that they would introduce a powder and have it right beside another of their offerings.
Unless they're phasing Magnum out...
The folks at Hodgdon should have something to say about your bullet/grand/caliber/combo