^ Agreed. A proper greyscale is probably the most important factor from my (novice) perspective. Once greyscale is correct, the picture can be made to look pretty good with some simple adjustments of the main color control. Not as good as a full CMS mind you, but significantly improved.
I've seen large improvements with my old Sony VW40 projector and with my new Panasonic VT30 plasma simply by adjusting greyscale in the SM and tweaking the color control by eye for a few hours using tons of different real-world material until flesh tones look "right". Doing this, I've ended up with a color value several points below what discs like S&M and DVE tell me it should be set at when using the blue filters. On both of these devices color was way too saturated at that level and it was readily apparent in most flesh tones.
EDITED TO ADD:
Also note that both of these devices had noticeably inaccurate greyscales out of the box in their most accurate mode (i.e. warm 2 on the Panasonic). On my Panasonic, "warm 2" was too warm and "warm 1" was too cool. This was apparent with most material before I ever hooked up a meter to measure. When the color temp is off just a few hundred degrees, a big difference can be seen in the resulting picture. For example, "warm 2" made everything a tad too red and "warm 1" made everything not quite red enough. They were each probably somewhere around 6000k and 7000k respectively instead of the 6500k goal. When I finally hooked up a meter, my suspicion was confirmed - blue was around 7-8% too low across the 30-100 IRE range while red and green were reasonably accurate (using "warm" mode in the SM which equals "warm 2" in the user menu). Boosting the blue drive in the SM alone made a significant improvement in the resulting picture (I had already properly set the main color control to reduce over-saturation on my display).
So no, I don't think that a CMS or video processor is necessary to see PQ improvements. Basic greyscale balancing combined with tweaking the "user" controls (contrast, brightness, color) can go a long way on their own based on my experience.