I have been running very tight 500Hz receive filters and for the most part they have been working great. However I realized that some stations are off my freq by 50-80Hz and the link becomes a problem as they are now getting one side of the spectrum cut-off. For stations that are off that far I have to run 600Hz filters.
Most stations I work have an AFC correction of no more than 2Hz but there have been a few stations that are in the 50-80Hz range on the AFC meter.
I always use 600 on my Flex and it seems to work well. I figured that some stations might be off-freq, so 500 would just be too tight. I also rolled off the filter sharpness.
I have seen this as well. I run my bandpass ony Flex slightly wider than 500 to allow for the correction but anything beyond 50hz seems to be a lost cause. I have recalibrated my rig a few times against WWV to ensure It wasn't me. Some older superhet rigs are just off a bit and VARA is pretty finicky about that. Rightly so with the number of carriers it tries to pack into that tight space
This can be a problem with drift and since VarAC defaults to FILTER2 in both my IC-705 and IC-7300, I have now set FILTER2 to 600Hz in both my radios and this way they are both good for VARA500 mode! Also Jose EA5HVK suggests that we use a narrow filter slightly wider than the VARA mode we use so we do not reduce the end carriers. For this reason I have added 50Hz on either side of the VARA signal and I have also set FILTER1 to 2400Hz which is good for VARA 2300 mode. BTW I still use FILTER3 which is set to 500Hz during RTTY Contests! Hi hi hi!!