Actually if you already spent a lot of money on a L lens, just buy the xs-pro nano. No point buying a lower grade one and keep on thinking what if I had spent that extra $20/$30 to get the better one. If your lens is only cost few hundred dollars, then maybe just buy the f-pro or go filterless.
Last time I brought in ksm xs-pro cpl before. There is filter thread for xs-pro ksm cpl. Only the old slim ksm cpl no filter thread. I remember not cheap wor, actually very expensive.