I think using or not using the factory barrel on donor guns depends on your circumstances.
I didn't because I chamber my own barrels, had a 32" 30 cal barrel and a new 300 Norma reamer in hand for another project. The release of the 338 RPR blindsided me, but I was very happy to see it and divert those parts. I was also acquiring parts for a 33XC project, so in my case why bother with a 26" 338 Lapua? I've done three 338 lapua based wildcats in the last couple years, so load development from scratch for the 33XC wasn't that big a deal.
My impression of the 33XC so far is jacketed 338 caliber bullets just aren't all that. I'm waiting for the new 338 ATip to come out before I pull the 1:9.3 barrel that's on it and replace it with a 1:7 for solids. I think the 33XC is going to be at it's best with the solids. The 285 ELDms weren't very consistent at long ranges, and the 300 Hybrids are just too much work to get right. The 2 boxes of Hybrids I bought have lengths and meplat diameters that are all over the place. I know guys that have made them work, but sorting, trimming and tipping makes cheap bullets expensive fast. I've tested the 30 caliber Atips in the 300 Norma, and they were unbelievably consistent at 2300 yards. I'm hoping the 338 caliber ATips will be more of that, but think the solids will still be the way to go with the 33XC. I'm wondering if I'm damaging the jacketed bullets with all the pressure the 33XC brass will take and will test that idea when the ATips come in. If that's the case, then solids will be the only way forward.
If I were new to long range shooting and wanted a good starting point with lots of data, the factory 338 Lapua RPR would be all that.