Hi Mike,...an XR6 is just a carburated 150...... they're tried and true and have been for years!!....They were the industry standard before the EFI's and the OptiMax technology took over. Think of it as the old muscle cars with the 350 v8's and holley carbs that we all loved before the fuel injection systems and everything else came along. You'll get excellent speed out of it, and they're easier to repair and work on than the new motors....the only disadvantage I can think of is you'll have rougher cold starts, and idling through shallow water will cause problems since your motor will be trimmed up and this can cause the gas to run out of the carbs and you'll stall.
Your gas consumption will be higher with that motor, but that all depends on how hard you run it and how often,..everything's relative y'know!! In summary, it's just a no bells and whistles straight 150 that will last for years,..maintenance and upkeep will be easy and I recently heard that Merc is only making a limited run of those XR6's that are available only on the NX896's.....and also available only to Tracker/Nitro.....but it's deffinetely a good motor,..it'll be fast and reliable,...just be prepaired to put a lot of gas in it!! They're thirsty!!!! Good luck!!