Somebody asked this last week about B+W filters. ......oh, it was you.!
The normal B+W filters fit under the DA15 hood but remember the slim version does not as it has a wider outer ring
There's another thread about this issue and one post mentions the Marumi DHG Super does fit OK under the hood.
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/79232-b-w-slim...tml#post802331
If you buy it I would still ask whether the outer ring has a wider "knurling" on the outer edge for gripping it.
I couldnt see any addtional light fall-off using the B+W standard (pretty thick) width filter even at f4 but if you use it for outdoors landscape in sunny whether you probably would be around f8-f11 anyway so no vignetting.
The centre resolution is great even at f4 and to the edges at f8 which is appropriate for outdoor landscape condtions where you need sharpness across the frame.
Shutter speed?
Well in bright sunny condtions f16 1/125 @ iso100 is the general sunny 16 rule an this is the same as f11 1/250 or f8 1/500. But remember you lose 1&1/2 to 2 stops of light with a polariser fitted on top of that so it might be back to f8 1/125
(Anyway I just use Av mode mostly at present)
Cloudy sky is 2 or 3 EV stops below this so at f8 it would be 1/125 or 1/60 at iso100. A polariser is of less benefit in cloudy skies unless you are reducing reflection from water etc.
If you want to, and like reading here is Luminous Landscape page on Polarisers
Polarizers
Remember you get a 49mm polariser for your DA15, it will also fit your DA35, FA43, FA50, DA70, FA77 and DFA100WR when you buy those too!