What will be the performance penalties of increasing the wing area of a delta winged aircraft to obtain a lower approach speed?
What you describe would probably work. In fact rate of climb and operational ceiling would improve. So too would maximum payload. But Mach 1.4 would have been a bad joke.

But your question reads a bit like "How can we improve on the Mirage III wing without doing all the things that were subsequently done to improve on it?"

One approach taken on the contemporary Convair deltas; F-102, F-106 and B-58, was to apply leading-edge camber with a conical profile, apex at the root and widening towards the tip. Developed by NASA, the technique significantly improved low-speed handling and, because it was designed to stay within the root shock cone, had surprisingly little effect at supersonic speeds. The second two designs were both capable of Mach 2. Even the F-102 just about reached your Mach 1.4, despite poor area ruling.