My .02: Own the 165 Supershape, weigh 165, find that even at freshly tuned 1/3 it doesn't feel quite as "grippy" on real ice as a number of other skis, including Contacts and Fischers, but it feels more planted, solid in the belly of the turn, so actual outcome is a toss. Huh?
My theory is that what we call "grip" is a function of initiation, edge angle, flex, dampness, and turn radius. First, we tend to find skis "grippy" if they hook up easily. Heads do not; they require strong early steering. Second, if you're not getting high enough edge angles, the ski will skid and/or chatter; this is a technique issue. Third, if you try to ski a perfectly tuned 14 m ski into a 16 m turn, physics says there will have to be some slippage, no matter what brand. And I think that in general, people carve bigger turns than they realize. So maybe you're just feeling that slight skid. On the other hand, if you try to bend that 14 sidecut into a 13 m turn, and fail because the skis are too stiff for the force you're generating from your weight and speed, you'll also get slippage, although for different reasons and in different places. But less likely IMO because most folks don't really bend skis except at higher speeds, and skiing fast with a 13-14 m cut is less common than with a 15-20 m cut. Fourth, the ability of a ski to absorb vibration will allow more of the edge to actually be in contact with the snow, so better grip. Vibrations increase with speed. Which is why real race skis are so damp and beefy. They need to be calm and damp at serious speeds. Heads, being very heavy and damp, keep more of the edges in contact with the surface than a light 'n lively ski.
So I suspect that your Magnums are too beefy and short radius for your weight, typical turns, and average speed. The i tech makes them stiffen even more as they flex. But OTOH, the dampness is nice on ice. So you have several choices:
1) Cheapest is to ski your existing Mags significantly harder and faster. Suspect you'll discover they "grip" better because you're applying more force, getting higher edge angles for more bend. But you'll also discover your legs are noodles by lunch.
2) Get some shorter Mags. Or Supershapes in 170, which are a bit softer (and most feel have more bite). 177 in either model is pretty long even for a guy your size. At 165 I could be happy on a 163 Magnum, a 170 would be for GS style skiing. Which is not always possible in the east.
3) Get a lighter, easier steering ski. 4x4's or Progressors come to mind. They'll hook up quicker and feel "sharper," I guarantee. Now whether that translates to actual performance on slope is another story, because I find both brands to be a touch lively for typical rutted hardpack and wavy ice. YMMV. Also, at the same length, the 4x4's are stiffer than your Magnums, so you may want to go for something no longer than 170. In the east, shorter is usually better.