I tried Collins and Inrad filters in TS870 and Inrad filters seem to work better. One difference is 400 vs. 500 bandwidth and it also appears that design and packaging has changed. I was surprised by the small size of late Collins filters. Maybe technology advanced and shrank the components. The only way to test or compare is to do some plotting of the bandpass curve for various filters in your radio and do some comparison.
In TS870 the DSP seems to skew the results, it contributes to the shape of filter curve - DSP seems to "improve" it and mask the true bandpass. The best thing is to measure the signal after the filter's IF buffer stage, before detectors and DSP stages. (Or use the network analyzer for those who have access.)
Sometimes even the "same" filters have different characteristics, manufacturing tollerances can play games here.
The best CW filter I have seen so far is the Collins 300 Hz 455 kHz IF crystal filter. Amazingly it has less attenuation than wider Kenwood and Inrad filters and very sharp skirts.
When considering replacement/addition of filters one has to look at impedances too. In some cases buffer amplifiers or matching devices should be implememnted and in case of cascading the same filters in particular IF, buffer amp might be needed to boost the insertion losses from additional filters. The most benefit is obtained from cascading sharp filters at the closest possible IF stage to the front end.
For the real rock crush proof front end there 50 ohm custom impedance filters available (INRAD) for custom design frequency. Great for those with their "own" CQing frequency. I haven't used them yet, but heard that some JA stations in metropolitan areas were using them.
Generally, if one wants to improve the receiver, the first thing should be to consider replacing the "stock" filters with better ones - gives the most bang for the money. Looks like some manufacturers are leaving this door open for upgrades by leaving enough room in the IF PC board stages area for extra
and larger filters (Yaesu, Icom).