Hmm, not sure what is going here. As far as I'm aware you can't spoof snrm on VDSL in the same way that you can ADSL. It is possible your router is just misreporting your speed in accordance with you changing snrm. Have speed test figures changed commensurately?
Also, although it sounds counter intuitive, I would have thought spoofing the snrm to a higher figure would fool the DSLAM into thinking you had a higher margin than you do and thus increase your speed rather than lowering it making the DSLAM think you hadn't sufficient margin and decrease your speed.
