Yep, I was basically saying that an inertialess drive doesn't make sense. Even if you confine the notion of inertia to merely momentum (so a momentum reduction drive), then the amount of force necessary to move the object would remain normal, but without momentum it would need to be applied constantly to make the ship move, hence a very low top speed. They would be able to accelerate to this top speed as well as come to a full stop instantaneously though. This is akin to an old sci fi movie that I can't remember the name of, in which some kids were inside an inertialess bubble and could stop and change direction instantaneously. They went to a space ship crewed by giant alien kids. Anyway.
However, if we're going to talk a mass reduction drive here then yes, the force needed to move, stop and turn are greatly reduced, allowing for high top speeds. However, I don't think the ships would be tremendously manoeuvrable. This is because they'd move such a great distance so quickly that reaction times alone would see a lot of straight line movement before executing turns. In this respect I think that the current AAF rules work to represent this quite well. I.e., 1 turn every 20cm. The distance is perhaps a touch too random though.
Of course, there is the possibility that the drive could be engaged part way. For example, mass reduced by 50% for normal fleet operations to increase combat speeds and manoeuvring. This could be used to give the Necrons superior speed and agility if so desired, either as a flat bonus or as a bonus to CTNH and BR. I'm inclined to think that this would come at the cost of hits though, but perhaps not, since it'd only be a relative reduction in mass, not an absolute one. Also, I wonder what the interaction effect would be when projectiles cross the fields' threshold. Perhaps they'd get a decrease in mass but an increase in velocity, changing the nature of their ballistic profile. Curious.