I can bet that once you get the 5 pound cable you will find out you've spent a lot of cash on unneeded thermostats. That was my story, but the thermostat for 2.4 20v is about 100 euros.
Now for the figures - the gauge and the real temperature have little, if anything, in common on my car. When the gauge reads 50, the ECU reads about 60; when the gauge reads 65 (halfway 50/70), the ECU reads 70; when the gauge reads 70, the ECU reads 83-84. I've done countless tests, every time with the same results. I finally gave up and just become worried if the needle goes above 90 on the gauge.
Conclusion #1: the gauge indicates a temperature consistently below what the ECU reads. This is fine, as it's the ECU controlling the fans (so, in case the ECU-read temperature goes above the limits (95 and 105 deg. I believe), the fan _will_ start).
Conclusion #2: the difference between the readings is _not linear_. The higher the temperature, the closer the two indications (gauge and ECU) are - when the gauge on my car shows close to 90, the ECU-read temperature is 90.
I also suspect the fan you're hearing is the high-speed fan, rather than the low-speed one. On my car, the low-speed is quiet enough not to be very audible.