"clip in" ND filters behind the lens - too much diffraction?

Hello Waterpixelers,

After seeing that a regular ND filter (even a shaved down one) will not fit inside my port with my lens, I started looking for solutions and came across "clip in" style ND filters that fit behind the lens, over the sensor which have the benefit of adding no extra length. The Kase ND64 is an example of this. 

When I brought it up, a respected topside portrait photographer told me that these clip in filters produce excess diffraction being so close to the sensor, and were in his words "unusable". Seeing as his experience isn't quite underwater, I was wondering if any of you had a similar observation. 

Currently I just let the ND filter sit on one of my double-flip slots and would really like to free it up for a second close up lens option.