20 minutes is plenty of time for the engine to fully warm up. A side effect of a bad stat is also slow and long warm up times. Your car should likely be fully warmed up within about 10 minutes of driving.
Side effects are poor fuel economy and extra mayonnaise in the crankcase. What happens the oil take a lot longer to warm up as compared to the coolant. So you risk not cooking water vapor out of the crankcase as well as the CCV possibly freezing up and possibly sucking the oil out of the crankcase.
You may also risk some accelerated engine wear due to slow warm up. I believe the typical operational temp should be close to 95C as I recall? I do not drive the E46 often, so I do not recall what the typical temps run. I will try to check my E46 sometime this week to confirm.