With my 77mm threaded 17-40 on the 5D I use the "Z" size filters (albeit Hitech ones now) but tend to hand hold them rather than use the holder, as the corners show! I also used to use a Sigma 10-20 on my 20D body and I think I wound up hand-holding the filters with that too! I only really use them with the camera on a tripod so for me it's OK. Your needs may of course be different. I have heard of some pros using blu-tack (is it called that in the US?
http://www.blutack.com/BLU_TACK.htm) as a makeshift holder!
The X-pro range at 130mm wide would work fine but i doubt the effectiveness of the grads. The Cokin filters are a "soft" grad and if the "fade" is in a ratio with the size of the filter, which it seems to be, comparing 'P' and 'Z' size (i have an older set of 'P' size that my stepson uses), then it's just not going to be effective. They're also expensive!
You can turn a Z pro filter holder round to just use one grad and that should be OK on a 10mm (16mm equivalent on a Canon "crop" body) I think, unless anyone knows otherwise? The Lee holder can also be configured to just use one filter. It also mounts Lee's own polariser in front of the holder, rather than a conventional screw in type stacked behind it (which of course increases the chance of the filter holder showing). I just think Lee are a bit pricier than they should be for what you get, personally. Not doubting the quality, mind! Having said that, Cokin's polarisers aren't cheap either! And they do of course mount in the holder.