It depends on the ski and the snow surface. On softer groomers, like we see out here, a 68mm ski isn't going to be necessarily a better tool than a 75-80mm ski. the edge width difference isn't all that significant. On real ice, it can be an issue, but the MX78 held just as well as the Speed Omeglass ti I was demoing the same day, on what I would consider full-on ice. Just a little slower to get onto edge. I find the MX78 to be a superbly versatile carver/all mountain machine. It perhaps isn't as good of a pure carver (in terms of just sucking you into the turn and tossing you across the fall line), but it matches the performance of bigger-turn hard snow skis (like a Fischer WC RC), and is such a good all-mountain ski when the conditions change. Most all-mountain skis don't have that kind of speed limit; actually, I don't know of another 176cm that is as stable as the MX78. Most pure carvers aren't in that range for bumps and crud, either. It is a different beast than the MX88, with the stiffer race stock phenol sidewalls (instead of the softer ABS sidewalls on the 88).
Instead of focusing too much on the width, I try and look more at the whole picture of the ski. There are plenty of skis in the ~70mm width range that can't hold the MX78's jock strap. As far as the Titan goes, the 170cm model I tried was much softer torsionally than the MX78. It was quick, snappy, and fun. The MX78 was a cruise missile in comparison, with more power, grip, and stability. The 177cm Titan was more comparable in feel, but much heavier, not as quick, more work, more tank-like, not that much fun. If you are looking for a narrower all-mountain ski with all the grip of a true carver (more than many, actually. I would say the MX78 outperforms a Blizzard Supersonic on hard snow); get the MX78. If in doubt, buy a Kastle Kti plate for it: that will beef it up even more. I think I still have one sitting around the shop (no MX78's though, except for a new pair that I am selling on consignment for a customer in 168cm).
Differences in width may make the ski less grabby off-piste, but yeah, the 78mm ski width won't float super well (well enough to work in 6-8" of new just fine, but not be "floaty"). For pure hard snow, nothing beats a 70mm or narrower width, but it can make a ski overly aggressive on hard snow. Which width is correct would depend on your personal situation. After skiing the MX78, I really don't have a desire for anything narrower: I like other skis just as well (Stockli Cross CX, Kastle RX12 for example) but that is because they are really fun skis, not necessarily because they are narrower. I just like the stiffness in the tail, the plate, and the overall pop of the RX12, which really isn't a function of the waist width; it has more metal and a thicker layup. Even some of the "carvers" are going up to 75mm in waist width: Fischer P900, Elan Waveflex 14.
Unless you live in a pure ice situation, you aren't going to find the grip of an MX78 lacking, or even that of the Head iPeak 78 Pro (provided they are well tuned). Sure, it might not be a full-on ice skate, but a good skier who can get good edge angle won't find it to be a problem on many hills. Now, I can't speak for those that live in heavy freeze thaw areas, or get lots of rain, but having skied the MX78 on seriously hard manmade (and injected) snow, it wasn't lacking.