I will also say that I skied my IM 78's yesterday (177cm length). Wow, these may be the best midfats ever made. I may have carved my best turns ever yesterday on these. They were just rocking on grooomers. Off piste they held up amazingly well, and with only a trace of fresh snow they were better and easier to ski than the K2 hardsides. When I found pockets of pow, they were still excellent and when i wanted to be agressive and straightline some stuff, they were incredibly solid. Great skis!! Terrible news that Head is dumbing down their peak series and taking metal out. Truly a shame!!
A small confession. This year, I owned and skied the Head IM 78 for one whole day. And yes you are hearing this from someone who calls others out for changing gear!

I probably broke Ron's record for least use of an owned ski, but, he came close with his Dynastar 4x4's this year. My reason for change was Philpug did an all-inclusive trade-in of the IM78 that enabled me to go to the MX 88. There was an extra cost to do the deal, and, my MX88's were demo's. Don't regret that upgrade, and, I'd do it again in a second. I really wanted a wider ski to take west. Still love the MX 88, it rocks!
Having said that, I would not have made the same exchange of the IM 78 for the MX 78. I really enjoyed my day on the IM 78 on an east coast mountain. I demo'd the MX 78 in similar conditions and thought it to be a great ski. I wish I had spent more time on both skis. Certainly the IM78 is now older technology, but, proven. I wonder if Head has a reason for the proposed change other than support of future sales. Head's marketing confounds me at almost all levels. If price was not an issue, the MX 78 would win, but, you get a small gain for a large cash difference. I'd rather put the bucks into an on-snow experience and ski someplace really neat. The limiting factor in ski performance is the driver of the skis, and, I've a long way to go. That's just one opinion, based on my needs.