OK Folks, just got off of the phone with Webasto tech support. The Air Top 2000 ST can be manually adjusted for high altitude by the end user.
It involves reducing the fuel supply as well, but does not need any other parts.
NEW Webasto Thermo & Comfort Air Top 2000 ST AirTop 2000ST 12v Diesel Air Heater. C $830.50; or Best Offer; Free Shipping. Webasto Air Top 2000 STC VW External Mount Air Heater Kit Diesel 12v Rotary Control, Installation & 1 Outlet Ducting Kit £ 899.06 £ 735.00 exc. VAT £ 882.00 inc. VAT This 2019 Webasto Air Top 2000 STC 12v VW Camper diesel air heater kit comes with a rotary controller, external mount VW Camper installation kit and single outlet ducting kit.