No, turning xvYCC on will accomplish nothing. xvYCC has to be present in the incoming video signal for it to do anything, In otherwords, the video has to be mastered in xvYCC for anything to happen. Turning on xvYCC and sending conventional consumer video will achieve nothing, you'll get normal old video because the consumer video stream is NOT an xvYCC video stream.
If the CMS controls aren't fixing an oversaturation problem, the controls are not working correctly... and one thing calibrators the world over understand is this: just because a TV has calibration controls doesn't mean the controls work right. Sometimes the controls are mislabeled. So before you give up completely, try other CMS controls to see if they affect saturation or not.
Also try the Color control to see if that affects saturation (up or down) -- some Color controls do, some don't, you can't assume what your control does until you makes some measurements before moving the control and compare those to measurements you make after making an adjustment. If your color control has a range of, say 100, and it is set to 50 and you are trying to increase saturation, I would raise the Color control by about 10 clicks for the second measurement so there' a big enough move to see on the graph in your calibration software. If the saturation does not change significantly, it's likely that your Color control is adjusting color luminance rather than saturation. That can come in handy when a TV has low color luminance and there are no CMS controls... you might be able to use Color to raise luminance enough to eliminate some of the errors. Also, don't assume the measurements you get in one picture mode (like Standard or Cinema, or Dynamic or Movie or Game or whatever) are going to be the same for all picture modes... you may find that 1 or more modes have undersaturated color, but some other mode has much better color saturation so measure all of them... though Dynamic mode is almost always locked out of being able to be adjusted to be accurate... most brands with a mode like that disable calibration controls when you select Dynamic mode.
So... if you need to decrease color saturation.... check all the picture modes and also check how adjusting the Color control up or down affects color (it might change saturation, it might change luminance, it might change both) and see if either of those help you out... and don't forget to adjust all 3 CMS controls for at least 1 color to see what each control really does to the color. Sometimes the CMS controls adjust hue, saturation, and luminance, but they might control 2 of those parameters at the same time. No way to tell without making measurements to see what's going on. Once you understand better what each CMS control is doing to the measured position for each color, you can better determine if they are going to be useful in fixing errors you are measuring.