The question really isn't if its less good than a fission gas rocket but if it could be as good or better than a conventional rocket. Could the gains in thrust offset the extra needed weight?
Gas core reactor rockets use nuclear gas reacting to super heat and therefore pressurize hydrogen. They operate at about 25000 C.
Why not use a high intensity plasma arc which routinely operate at about 13,000 C but if designed to can go much higher by at least several fold.
I got bored...