Will be interesting to see if you get your 5lph from the D6. My Ivecos are 7.5lph at 8 - 8.5knts and around 6lph if I keep it back to 7.5 - 8knts. My limited experience with a new D6 we installed a few years back didn't suggest to me that it was any more economical. Might get a small gain from the trimmable leg vs an angled prop, but 5lph at ~8knts would be a very good figure.
Agree that the Ivecos aren't any good for a leg with the high torque and I wouldn't have done the Konrad thing either. In that 370hp range the D6 is probably the tidiest package as you say, but buying Volvo's is what I have found to be an expensive long term exercise. One good thing though is that once you build up your network of alternative parts suppliers and overseas contacts you can avoid some of the rape and pillage that is usually associated with buying Volvo parts in NZ. If you do all your servicing through a Volvo dealer you probably won't like the numbers very much.
Did you work out how far off your trolling speed the supercharger kicks in? On our KAD32 boat you would have to be on the superchargers to troll at 8 knts and I can't see you getting your fuel numbers with the supercharger running full time. If you are on the cusp of the supercharger at trolling speed you are going to hate it as the charger kicks in every time you surf down a wave, meaning throttle changes and stuffing around to get back to the correct speed again. Stupid bloody system really. Just finished installing some supercharger cutout switches in a boat yesterday for exactly that reason. Couldn't run the thing at slow speeds in big seas without either going so slow that the boat was hard to control or so fast that the superchargers kicked in on every 2nd wave and slammed the boat into the next wave before you could pull the throttle back.
Why did you decide on a leg? Do you have shallow water requirements, going on a trailer, or is it to do with the hull layout? Probably would have opted for a shaft in a tunnel myself if possible. The later V drives are much improved on the earlier ones if you still want the engine under the cockpit.