A 5mm difference in width will be noticeable if you're used to swapping and changing skis and/or you demo a lot of skis. The flex and, by implication, the grip of the ski will have more of an impact under those circumstances (especially on a European piste) although there will be a very slight difference in how quickly the ski rolls from edge to edge. A difference of 90mm in length will be noticeable for most skiers if they were to ski the two back to back. If you're starting from an ideal point in the middle a 45mm change longer or shorter is manageable - it's just an inch or so at tip and tail and you quickly get used to it. If you're stretching almost a full size to a longer ski - 80mm, 90mm or 100mm (in the case of Kastle) - it would take a bit more effort to get used to the ski. Depending on how confident you are on the longer ski it may not suit you at all, or you may become accustomed to it as you progress. The question then is whether the longer ski is inhibiting or helping your progress. Some people can't settle on a length at all with certain lines in the Kastle range finding one too short and another too long (or both too expensive).
I'm 6'4" and 210lbs. I demoed the 176 MX78 three times and loved it for zooming the grooming. When I visited the States a couple of years back I found a pristine demo pair in 184, thought about it for maybe six seconds, and bought them. I'm a bit better skier now than I was when I originally demoed the 176, but I'm happy with the longer ski. No doubt I could have been happy with the 176, but at my size it would have been a turny groomer ski. That's what really excited me on that first run; turny, smooth and stable, but still providing good feel for the snow. Overall I think I'm happier with the 20m radius and more GS feel of the 184. I have other skis now for days when I want to rip some slalom turns. Before I bought the Kastles I would have put my ideal length for this type of ski at about 180cm, so I was in the middle there. The 176 was a tad short and the 184 a tad long. [edit - the 176 to 184 gap of the MX78 is the only 8cm gap I've yet seen from Kastle ... curious]
The real questions are how do you fit with your 167 Atomics, and how do you prefer to ski? That Atomic D2 VC is a pretty different animal. My friend demoed them a few years back over the course of a week. The Vario Cut (I called them the "Camel Toe") was a totally different approach. I recall the tips were relatively soft and the flex / side cut would shorten up remarkably as you left the hard snow and ploughed into something softer. They'd suddenly just turn harder without any added effort. Took some getting used to.
I think any of the skis you're looking at would work for you - the 158 MX78, the 163 MX83 or even the 168 MX78 - and it will come down to whether you prefer to throw in a lot of turns (shorter MX78), hoon down the slopes GS-style (longer MX78) or do a bit of both and occasionally dive off piste or into the bumps (MX83). There's a lot of overlap between those three skis and you can hardly go wrong choosing between them. Do try hard to demo them to make sure you love your final choice. And, once you demo the MX series, you'd better have your credit card handy. They're that good.
Best of luck.
Edited by sinbad7 - 2/24/14 at 3:26am