If I am a decision maker in SM motors, or in fact any distributors in singapore, I would be hesitant to bring in any cars with a V8 engine when the same model has a V6 smaller cc engine. Singaporean mentality is always a biggest car with the smallest engine possible.
Therefore I always think that the process of launching BMW 6-series is excellent in the interest of performance motors. Launch the V8 645 first, then a few months later than launch the 630. People who like the car so much would probably go for the 645 while after the demand has slowed down, then come out with a cheaper model to boost demand back. I would say if is applicable only to those really unique or new models / conceps which are really attractive.
The XC90 falls in the unique catagory of a seven seated SUV with bascially no direct rivals. Other cars that I can think of in this category is the Toyota Fortuner and the Suzuki Vitara XL-7, both of which are exactly close competitors to the XC90. I mean if you are looking at XC90, you would probably not consider the Toyota or Suzuki at all.
Therefore personally I think they should bring in small volume of the 4.4 V8. Picture this, imagine I have all the money in the world and I want a car with that is a SUV as a statement but can carry more than 5/6 comfortably, the only choice I have is an XC90 (Sorry...Cayenne Turbo out becos can only carry 5...

) and I would probably go for the top-end V8 model, with probably a smoother and more powerful engine as well as load of equipements.