I agree that it's pointless to apply two separate colour filters, one for RGB then one for the P3.
On that note, I came across this interesting German site showing they can boost contrast by 40-50% simply by adding a filter tuned to correcting the colours in the highest-measuring contrast mode:
It makes perfect sense, as if you lose 40% of your lumens to correct the colours digitally, to the most accurate mode, then that must mean it's done by limiting the max brightness level of each primary to some degree. So if you use a filter which allows you to simply use the full bit depth range of each colour channel, then you've boosted the contrast since you can go brighter, but meanwhile the filter actually reduces the black level, hence the contrast is boosted by the same percentage that calibration loses it. Lots of DLP projectors start off at 2000:1 then in the best calibration mode drop to 1400:1 or less. It's also interesting how they put the filter at an angle to avoid shadows being reflected back into the primary lens, which is curved.
Moral of the story: always avoid correcting brightness digitally when you can, it reduces bit depth and contrast. But thankfully you can gain that contrast back using an additional filter.
A smart move for projector manufacturers who are torn between having good rec 709 vs P3 contrast performance and lumens, is having the projector be Rec 709 native at the colour wheel but have a P3 filter that can be clipped on, outside the projector. That might be less convenient than having a filter mechanically in the light path, but many Epson owners had problems with their Cinema filters breaking and it adds costs and inconvenience when you have to send in the projector for repairs. An outboard P3 filter is a good idea, I think.
would make some contrast-preserving filters for rec 709 DLP projectors.