I've got the cw filter in my '817, and the SSB filter in my '857. #Here's some explanation about "why?" for the SSB filter...
Document has graphs of the two filter responses
[edit] The article is primarily focused on the benefits of W4WB's mod to put both filters in the '817, which is irrelevant to the '857, but there's a good discussion of the technical merits of the SSB filter.
As you can see from the figures in the link, the Collins filter is flatter and has steeper skirts than the stock filter, but it's a little bit wider. #To answer the question, "Is the price of the filter in line with performance inhancement?" I'd say the answer is...it depends. #The Collins will give you a signal that, under adverse conditions, will be heard better than the stock filter, but the conditions have to be pretty bad for that to be a noticable difference. #Same goes for receiving.
But, the Collins
does improve the "sound" of the received and transmitted signals. #And, under good signal conditions it's a matter of your personal taste. #I hear the difference when I switch back and forth between the two filters, and I'm a died-in-the-wool fan of Collins Mechanical filters so it's worth it to me.
The good news is that it's easy to install later, and you need not hurry your decision.
As for the CW filter, it's a required option for the '817, because it's the only way to get usable selectivity in that mode. #With the '857, you've got the DSP filter, and that does a good job narrowing the audio response. #That helps a great deal in a lot of cases, but won't handle the case where you're trying to pull out a weak signal with a strong signal in the SSB filter range.
I think it's reasonable to say that if you're planning serious CW work, you'll be glad to have the Collins filter. #BTW...I believe it comes in two flavors, 300 Hz, and 500 Hz. #The advice I've gotten is 500 is for CW, 300 is for RTTY. #300's a little too narrow for CW, except under the worst possible condx.
Hope this helps. #I haven't had a lot of chance to use my '857, but when I have, I LOVE it. #Have fun!
Also, if you don't have the FT-Meter, get it ASAP. #I wouldn't have an '857 without one.!
73, and enjoy!