I've experienced the same problem shifting my G-35 6spd. The revs don't fall quickly enough between shifts. While upshifting, each higher gear is normally engaged at a lower RPM than the peak of the prior gear. The clutch does not engage smoothly if the RPM is too high or too low. In this case it requires a slight wait for the RPM to fall to the right sync RPM. I once had a Honda on which this RPM drop could be adjusted. Don't know if the G-35 can be adjusted. I suspect everything is computer controlled now.
Also, my car has needed a very long break-in. At 2,300 miles now, there is a dramatic improvement in engine power over new ( with 10 mi).
Local Infinity dealer mechanic says the engine will continue to improve to 5,000 miles