Is there any worries about the filter not being 100% parallel to the back element? If it is not totally square would this serve to add some distortion to the final image. I guess not since you find it successful. Have you done any A/B studies with the filter on the front, then back then no filter to see if there is any degradation? I suppose I'm getting to nit picky, something I try to avoid when it comes to my photography. I find some get more obsessed with techie kinds of things and not enough with the "art".

Eric