My observation is that this vehicle - 2005 Range Rover HSE US version - that has M62 v8 engine is getting 15 mpg at max on combined city/highway cycle.

Is this normal/expected?

Not compared to the Non US version, you would see around 20mpg on a long run cruising around 70 mph in the non US spec cars.

However, US versions have a lot more anti smog gear esp california cars as I understand it and the US gallon is 3.78 litres vs the UK gallon @ 4.54 litres.

So at a guess I'd say 15mpg in the US is probably about average. Fuji White / Jet 2012 4.4 TDV8 Westminster

In my experience a combined cycle consumption of 15,7 lt/100 km is typical / normal for this vehicle. This translates into roughly 18 mpg (UK) or abt 15 mpg (US). An increase of 0,5-0,6 lt/100 km should be expected with full Air Conditioning during the hottest months.
Motorway cruising at around 110-120 km/h corresponds to 12,5-13,5 lt/100 km

I usually get 16.5 average mix of shorter journeys into town, and around 22 on a longer motorway run sat around 75

That seems about right ^^