Samsung is in the fab business and qualcomm is their customer. Qualcomm could easily go elsewhere and Samsung would be stuck with producing only their chips, driving up their cost. Anyway, all this is just rumor and hearsay so we'll see what's what in a couple of months. But, considering that Samsung is the only firm complaining about the S810 overheating, I think this may be an excuse to ditch Snapdragon for their own subpar, bottom-feeding SoC and, hopefully, stay in the good graces of Qualcomm. Or maybe it's a way to pressure Qualcomm in lowering its price. All speculations at this point...
Meh - it's not speculation that the S810 won't be custom cores. It's not speculation that Samsung has experience with the A57/A53 in phones that launched 4 months ago on their 20nm process.
I'm personally leaning toward the S810 being underwhelming in 1H 2015. Ever since QC released the S800, it feels a bit like they've been coasting with the S801 and S805 being minor improvements. Heck a LG G2 using a S800 released in summer 2013 still feels competitive performance-wise to QC's current stuff.