I've owned and/or skied all of the Head Monsters. These are pretty quick and easy from edge to edge for a higher performance 78 mm ski. If you've mainly skied 66-68 mm, they may feel a bit slow. They're also not ice skates compared to the skis you're used to.
But two ideas: First, you don't state your size or the ski length. iM78's are fairly heavy (three sheets of metal your year) so could be that they're too long for you, will feel unwieldy at slower speeds. They have fairly soft tips, so they should really leap into turns (for that width) if you have the right length. Their best trick is the i tech, which IMO really works; you can feel them stiffening up as you gain speed.
Two, you don't state if they were new (unlikely). But whether new or used, they are probably out of tune. Heads are notorious for having lousy factory tunes, and IME, most big box ski store tunes are so-so at best. Take them to a reliable shop at a slope, and have them hand tuned to 1/3. You'll be surprised at the improvement. I skied 171 iM78's as a demo and thought they were really fun, but mediocre carvers. Bought a pair of 177's cheap on the spur of the moment (go figure), had them tuned by a specialist, slapped a plate on them, and they handle northeastern boilerplate just fine. (Well, not like my rec racers, but as good or better than any other 76-78 mm ski I've tried, including some Fischers.) And they're amazingly versatile - try them in bumps or variable soft to begin to appreciate what they'll laugh at that your SC's hate. You actually stumbled onto a classic model, incidentally, with a somewhat stiffer sidewall than the next season's, that really likes to motor through bad snow. Read Dawgcatching's several reviews of the 171 and 177 to get the idea.
Can't speak to the Rossi 82's, have heard they're a fine ski. Suspect from the reviews a very different feel, though; Rossi's are lighter, not as damp - and have heard they're more a forgiving GS-ish groomer carver with a slightly lower speed limit, while the iM's will be more of an all-mountain SL-ish carver, if that inversion makes any sense. Simpler version: iM78's will be more versatile and have a tighter response, not quite as into carving big round arcs on ice, better maching crud. Hope this helps.
Edited by beyond - 12/27/10 at 9:18am