I had the same problem as you with my IIYAMA A201HT, VisionMaster Pro 510.
in fact, the new drivers are using EDID infos given by the monitor and the nvidia driver use the higher rez it compute using that not really accurate data.
My monitor is detected having 400MHz bandwith (360 in reality) and HorizSync 30.0-100.0 VertRefresh 50.0-130.0 (in reality it is HorizSync 30.0-130.0 VertRefresh 50.0-160.0).
So I put in the device section of the xorg.conf file :
Option "UseEDID" "False"
And now the nvidia is forced to use my own modelines !