Just had a Webasto Thermotop EVO 4kw installed for 2 days now, and it's all i hoped for

it's petrol powered from the EHS own tank and installed so it heats the inlet side of the cabin radiator (heating the cooling water) It's controlled from an app on your phone.
After it's been running for 20mins prior to driving i can drive away in EV mode in zero degrees celsius (been down to -3 which was also fine) with a 20 degrees preheated cabin, and if i keep the heater running when driving it keeps the cabin warm & Engine off

there's no warning lights or any other signs of the car not 'understanding' the preheated engine, it's almost like the car is designed to work like this.
the heater uses the built in 12v battery, which is recharged along the HV batt.
petrol usage is around 0.3L /h
the shop that installed it was quite surprised about the build quality of the EHS, he said it's really easy to work on and construction/assembly is on par with Japanese brands, not the worst to be compared with