hmmm,, MIke, I think you need to re-test the MX78, there's no comparison. Seriously.
It's funny how we perceive things. I remember Phil asking me how I liked the MX 78. I just made this big grinning face. Wow, great ski. But, I also remember skiing the IM 78 really hard all day and I did not find it wanting for anything in conditions I ski all the time. At the end of that day, if anyone told me I'd be trading it in on another ski, I would have said "not going to happen!". Never, not even for a second, did I consider upgrading to the MX78. That's much more of a personal statement than it is a review of either ski.
Reread John Botti's thoughts about the IM 78, a ski that is way-old by today's standards and does not generate any hype. It sure goes against the grain of ski marketing. How well does a "best" ski help over a "really good" ski? I leave that decision for each to decide according to their needs.
The Kastle's you, G and I bought helped us do quiver adjustments to support perceived individual needs/wants.
In a similar vein, sure, I'd like to trade my daily driver Supershapes for a RX 70. But for what advantage? I can hear Phil smiling at this marketing opening.
But that's just me. Not trading in my golf clubs or road bike either. Need to ski more to ski better!