So I guess I'd say, as far as real improvements vs. just trying to sell stuff, it is probably a bit of both and a bit of neither. The personalities who drive FX genuinely seem relentless in their drive for, and pursuit of, performance improvement. I'm sure some of that feeds their sales, because there is always someone who has to have the "latest and greatest," but I think a lot of it is just because that is how they think and what they do. As far as manufacturing, support, inventory, dealer relationships, etc goes I'm sure it is an absolute nightmare. How many variations on every single part must they now have to inventory, stock, be able to identify, track, and support? Messy and very challenging, I sure don't envy them. Also development time is VERY expensive, so the fact that they're willing to make real changes I think speaks to them doing it because they genuinely feel it improves the product, not just trying to make money.
As far as whether there is a "real improvement," I think that question really has to be countered with "to who exactly?"
A lot of the changes for the MK II impact were done to improve its power ceiling. As slugs and larger calibers are in vogue, people demand more and more air to sling them at higher and higher velocities. So if that is your game, then it definitely is a substantial design improvement. However the thing I always say to people who are upset because they just bought a _____ gun and suddenly it is "obsolete" because some new tweak was rolled out, I'd point out that your gun is every big as accurate and good to shoot today as it was yesterday. If it was good enough for you then, it stands to reason it is still good enough for you now.
The one big change, I suspect, between your impact and the newer ones are the X barrels. Most people love the new X barrels, a few people swear by the old ST barrels. If you don't have any accuracy issues I see no reason to change, however if you do want to try a new X barrel they have an exciting variety in different diameters, twist rates, etc available at very reasonable prices. (generally about 100$ per barrel) So it should be an easy swap if you'd like to give it a go.
Just my 2c on the whole thing, as a guy who both really appreciates that FX is continually improving things and developing new technology, but who also is largely happy with his rifles as-is and doesn't feel the need to upgrade every time a new doodad rolls out. I hope that helps.