The point is that the RB25 engine and box is a much better starting point than an RB20. The extra torque would more than compensate for the added weight.
They seem to more than hold their own club level events in Japan as well, along with C34/C35 Laurels, Cima's, Aristo's and the list goes on. Night Walkers JZX100 campaigned in D1 for over 2 years at close to 1600kg which is HEAVIER than stock.... Weight isn't the be all and end all.