I don't think they will us 78Kwh battery pack again. So it will not be P8... should be P10 or higher number.
But... unless they improve their efficiency, I don't think people will buy it. From SPA2, they should improve it a lot.
My understanding is that the number "5", "6", "8" in Volvo model names has to do generally with horsepower: generally comparing to an inline 5-cylinder, or V6, or V8 ICE, respectively, with their supercharging, turbocharging, and electric-motorizing of their tiny 4-cylinder, 2-liter engines. The "8" in P8 for the BEV suggests a power level like that of a V8 ICE, I'm pretty sure, so it won't go to "P10" because the power won't increase -- only the range will increase when they increase the size of the battery pack.
I disagree with you on people not buying the current XC40 BEV because of efficiency. The vast majority of BEV buyers don't do road long trips in their BEVs: they charge only at home (maybe also at work, if that's a possibility) and stay within a full charge of home. Since the average American drives about 29 miles per day, a 150- to 200-mile range is perfectly fine for most BEV drivers (most BEV owners use an ICEV or PHEV to drive on road trips). I have graded all of the BEVs for sale in the US in 2021 that I know of (in terms of utility, aesthetics, and quality), and the XC40 ranks in the top three of that list for all-around marks (along with the Audi E-Tron and the Kia Niro). I'm pleasantly surprised because I have given up on the current batch of ICE/PHEV Volvos due to their crappy engines, horrible all-electric driving range (PHEVs), and the Sensus computer system; the XC40 P8 benefits by not having an engine, having decent electric range (as I've defined above), and having replaced Sensus with the Google OS.