Using the in-lens filter is likely to significantly reduce your effective resolution. The Bayer filter in front of the sensor registers Green, Red, Green, Blue across the sensor, a Red filter will (depending on strength) reduce the sensitivity of the Green and Blue pixel positions the camera needs to interpolate the remainder.
Mono processing in camera will give much better results than a filter on the lens.
---------- Post added 01-05-20 at 05:59 PM ----------
Here's an example with 100% crops from a 645D using the P6x7 35mm Fish-Eye with and without the red in-lens filter (without conversion to mono):
Last edited by johnha; 01-05-2020 at 11:08 AM.