keep in mind the other limitation is how much current the battery can comfortably flow out along with traction considerations.
From a marketing perspective, and Rivian is sharper than most here, they need to leave headroom for an R2 Tri which I'd guess would come in around 800hp. So capping R2 Dual performance nearer 650 than 700 leaves enough of a perceptual difference to justify a decent upcharge.
Mind you, I write this as a Dual Large owner who paid for the Performance unlock before the Gen 2 Tri and Quad came out. I did test drive a Tri before taking delivery, but there wasn't enough in it for me to justify the upcharge. YMMV
It is interesting that the torque only goes up 70lbft and yet the time drops 1 full second. Way more of a drop for such a small increase in torque compared to RWD vs Premium.