A few things. If an impact requirement is imposed you have to requal to increase heat input. If not, you can revise. You have to revise because if the WPS says heat input control you have to do heat input control.
Also, 1.5 kj/mm is actually very low for SAW. Thats normally about where I'd prefer my GTAW.
Also, there is a fundamental misconception that higher heat inputs necessarily cause lesser impact toughness. This is mostly true of single pass welds where grain growth is a more predominant phenomena. But in multipass applications quite often the increased heat input can penetrate deeper into prior beads and actually normalize a larger volume percent of the those beads and thereby improve impacts from the original deposit. But too much heat input and now the grain growth HAZ of prior beads starts to enlarge and predominate and again reduce toughness. Good heat input control is actually a window not a maximum.
Anyone dealing with duplex stainless steel is painfully aware of this.