I just found this thread and very much appreciate Mr. Green working to get some pressure data
On the other hand I can get Lapua 284 Win brass and could simply go the tried & proven route.
The question of "Why not the 284?" has come up several times in this thread. On one of his YouTube videos, Erik Cortina said that his 284 ( a Shehane, IIRC) was just as accurate as his 7PRCW. In fact, he said he carried the 284 to South Africa for a competition as a backup rifle. According to a couple of comments in videos, Cortina said that he switched to the "magnum cartridge", 7-6.5 PRCW to get better brass life. The history of the 284 Winchester is largely to blame for for this.
Short Answer: The 284 Winchester is a 56K psi cartridge (the 6.5-284 is 58K psi) and some shooters routinely push it above 65K where brass life is not good. The 6.5 PRC starts off as a 65k psi cartridge, so the brass is made to handle higher pressure.
For what it's worth, in a Hornady podcast, it was mentioned that several common competition 6.5-284 loads were pressure tested at above 65K psi. The 284 competition loads are likely not lower.
Longer Answer: By the early 1960's, Winchester wanted to compete against the Remington's Models 740 (semi-auto) and 760 (pump action) rifles which could chamber the 280 Rem, Remington's answer to the 270. Winchester responded with the Model 100 semi-auto and Model 88 lever-action rifles in 284 Win. Those rifles were originally designed around the shorter 308 Win cartridge. As is typical with those types of actions, the bolt strength is the weak link and they were designed to handle the loads of the 308. To get 270 or 280-like performance, Winchester got clever and developed the 284, which had a fatter case than the other two (0.500"vs 0.470" dia). They then rebated the rim to match the 308-sized case head. Bolt strength then became an issue. The backthrust against a bolt is related to max case (internal) cross-section area multiplied by the max chamber pressure. To match the bolt thrust of the more narrow 308 at 62Kpsi, the SAAMI Max Average Pressure for the 284 was limited to only 56,000psi. The later Norma 6.5-284 is only slightly higher at 58K psi.
Yes, it would be possible to make 284 brass that could handle higher pressures, but that would involve thickening web and the walls near the base. This would reduce case volume and velocity for those who don't need the extra strength. The 6.5 Grendel and 6mm ARC are in a similar predicament, BTW. The brass was thinned out to provide max powder capacity at the relatively low 52K psi needed to avoid bolt problems with AR15's. This, however has prevented its use at higher pressures in semi-autos with stronger bolts (bolt-actions with chambers that offer more support near the case head can go a bit higher, though).
For those who really want to stay with a 284 derivative and run higher pressures, there is a potential solution. The relatively new 6.5 and 338 Weatherby RPM cartridges use a longer variant of the 284 case that is beefed up to handle 65k psi. It would take cutting this brass down, forming and neck trimming, but the case life should be better. The propellant volume will be lower, though.