It's not just that Lapua brass is better brass. It is, but the 338 Lapua brass in particular is manufactured with a very thick web that will withstand a bit higher pressure for more firings than most brass. Many believe 338 Lapua brass to be among the toughest, if not the toughest small arms brass available. Other Lapua brass is not made with webs as thick as 338 Lapua brass, from what I've seen. In other words, if Shawn made a 338 Lapua +P, taking full advantage of case capacity as well as the throat design employed, my guess is it would perform similarly, but last longer than 338 EDGE +P brass does now.
I commonly get 10 or more firings from the 338 AX (338 Lap Imp) chambering, which is probably a little easier on brass than the standard 338 Lapua with its ~35 deg shoulder. Just my non-expert .02.
I'd like to see what a 338 Lap Imp +P would do--might be just on the edge of eroding the +P throat too fast to make it worthwhile to try, which would put you right back to the 338 EDGE +P as the best combination of firepower and throat longevity. Not sure. Speculation on my part. Shawn would obviously have a much better handle on this.