I'll second that, Matt.
I got my cable today and a fresh copy of FiatECUscan (mind that I drive a diesel, Alfadiag is probably the one to go for on a TS) and I've discovered that my 70C on the gauge are in fact 78-82 C according to the ECU, which is the exact opening interval for my thermostat. I would go as far as saying that "good" gauges on JTDs are in fact factory-miscalibrated to show the familiar image of 90C rather than the real 78-82 which appears to be the standard operating temperature for diesels. So while my gauge shows 10 deg below the real temperature, the "good" ones are displaying around 10 deg above the real value.
The secondary result of today's check - I realized I probably spent 100+ EUR to replace the thermostat for no reason at all, as the previous one was sitting at 70C as well. Well, I should've gotten the cable first.