Even though I think the 47 Lapua is a better case design, already having a 20-250 that shoots the Berger 55's extremely well, if I had all to do over again I would probably have a hard time switching to a 20x47.
Since you plan on only shooting lighter bullets the 20-250 is already a lot overbore and going to an even larger case, more overbore, can sometimes be to much of a good thing. If you were wanting to sling the 55's then I'd probably be more compelled to go 20x47 Lapua.
It's a shame Nosler has never developed their 20 Nosler in to production because a 20 cal that had a little more case capacity than a 20BR but on a small bolt face and would feed slick would have been a really fun little cartridge to have, especially for slinging the 40 grainers. But Nosler isn't known for having the greatest brass so we probably would have been left with disappointment anyway.
So! Since there are quit a few of us on here with 20-250's and nobody seems to have a 20x47 I vote Austin Laughlin has a 20x47 built. If for no other reason, just because. There, now it's settled and if for any reason it doesn't work out you can blame me.
I know you love those lighter bullets but FWIW, a 20x47 using RL26 out of a 26in tube should hit 3800 with the Berger 55's and if you look at the BC of those 55's they are like a long range ballistic missile. They're about the sleekest sexiest looking bullet you've ever seen. The dam things look like they are going mach 5 just sitting there. Oh, and you'll want at least a 9tw barrel.