Well you certainly cannot rely on the Bayer matrix to do the filtering. On a Lumix I own theres enough leakage of light to take a photo during the day with a filter that cuts off light at a shorter wavelength than 820nm. Green lawns look like snow!
In the UK – given its dirty and humid atmosphere – hard UV certainly isnt a problem (well, it didnt seem to be for me) though the definition of where the UV starts and blue ends may vary and the big worry is how fast a refractors focus goes to pot as you move toward the blue – may get bad sooner than you think.
But even with reflective optics, the colour balance will still be off as the blue and green images will still contain a contribution from the near infrared – that can blur out fine contrast features.
Personally, if the camera already has a blocker but you have a professional quality blocker to hand its worth experimenting using the camera with both filters, your professional filter only and with only the in-camera filter and comparing. You do have to be very certain of your filter quality.