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If you say so. I have the graphs here that say you are misinterpreting
what you are seeing and hearing. What you are calling blow by of CW
signals is indeed ringing. It is a vicious problem with sharp filters.
I spent quite some effort for a fast frequency hopping radio getting
the ringing down to levels such that I did not experience jamming
by strong adjacent channel signals simply due to this basic property
of filters. A softer stop band curve will show less ringing for
signals outside the passband. It will snow less attenuation of pure
continuous carriers. At some point there is an optimal compromise for
your given application. I note that the basic ProII filters are VERY
sharp. As such mathematical analysis of the filter properties suggests
you will get filter output excited by signals outside the filter's
passband and that this will be an annoying problem in most cases.

It will be an issue with any transceiver that has a very sharp
CW filter. There's nothing that can be done about it.

As an aside what engineers call blow by is actual signal leakage
around the filter when tested with continuous carriers. The digital
filters appear to have precious little of this blow by. The filter
lobes are far enough below the passband level that you're talking
of a fantasy world with better filtering the last I checked on my
ProII.

I really figure what CW operators call blow by is something entirely
different, filter ringing due to excitation from off frequency
signals. The more "key click" the off frequency signal has the worse
this ringing will be. And note that the ringing will not necessarily
be classic 100Hz single crystal filter ringing that happens with on
frequency signals due simply to the narrow pass band.

If you want to use filters correctly in weak signal vs. strong signal
environments it does pay to understand your filter characteristics
very intimately so you can make useful compromises.

{^_^} W5MKU, who has considerable experience designing exotic
receivers and frequency synthesizers usually for military
applications.