You might conclude that the only real option is to buy a positive pressure fresh air system, but this comparison doesn't match my experience. I have no trouble driving the PM2.5 very close to 0 with a few standalone purifiers in my house (verified with a couple air quality monitors) even during bad wildfire smoke events. I'll grant I live in a modern house which is pretty well sealed, but it's also a question of sizing the purifier to the room's size and drafts.
There are some orthogonal advantages and disadvantages though. On the upside, you'll reduce CO2 and other pollutants from inside your house -- it can get quite stuffy when you can't ventilate due to poor outdoor air quality. A downside is that it may also bring in air at a different temperature or humidity than you'd like (though that can also be controlled to some extent with a HRV/ERV.)
I think you've mixed up relative and absolute differences. Both of your examples are in terms of absolute percentage points (pp, sometimes ambiguously labeled %), but relative changes are measured in percent (%). 300% relative difference just means 300/100 = 3x.
There are some orthogonal advantages and disadvantages though. On the upside, you'll reduce CO2 and other pollutants from inside your house -- it can get quite stuffy when you can't ventilate due to poor outdoor air quality. A downside is that it may also bring in air at a different temperature or humidity than you'd like (though that can also be controlled to some extent with a HRV/ERV.)