Mach 3.5 at FL800 is about 250KIAS, not so bad.
But temperature raise is definitely a factor.
But new generation aircrafts had been always build after new type of engines. This model lacks a idea for engines. The so called CDE would be a plausible idea. The idea is constantly to have small explosions which producing a substantial higher impulse than rocket motors. Possible would be a marriage of ramjet with a CDE motor. If the small cannons are in positions to bring the airstream into rotation, it could be a highly efficient motor.
Aerodynamically all of this drafts are utterly weak, producing high drag, low lift in certain situations, low maneuverability. They showing clearly that since several years no inventors are in Aerodynamik.