Thanks for the interest all.
Sadly I don't have a schematic for this circuit, although if I get very keen I may trace it out. I imagine that the circuit is the classic type of analogue multiplier involving logarithmic conversion and then back again - although I've not got my head around how/why you would use a large number of identical devices to acheive this.
Spec, I'd not though of measuring the curve, but yes, I suppose that's what I'd need to to to ensure a good match. In addition to your comments about bulk resistance (something else I'd not thought about), I suspect that they also opted for a medium power package with a thermal pad for good thermal coupling - the banks of devices are mounded 2-sided on a ceramic substrate.