Not going to question what JBR is saying since they're selling a product, but note:
-The "oil" that is left over on the intake manifold can be found in most engines. The kicker- most of it is volatiles, including water and gasoline from the blowby. If you leave it alone a few days, most of it evaporates
-The backside of the valves don't really show deposits. That's stuff left behind by gasoline (they should have also shown the intake port walls). Oil deposits actually look more gummy and is an actual buildup over time. Like this (maybe less severe):
-If the Mazda 3 actually did have a PCV oil consumption issue, it's going to be a lot of work to fix it. Back before 2012, you often find cars with fairly primitive oil separators. These are meant to knock out larger oil particles from air. Most simple catch cans work. However, Direct Injection has resulted in already better separation, and requires REALLY fine oil separation in catch cans to make a difference- mostly the filter/fiber type. You're mostly knocking out the volatiles and not the actual oil, otherwise. Check out their last picture of the separated components- and that's a picture from a Mazdaspeed 3, a much older engine without as efficient a PCV system, one that has more blowby due to the turbo, and requires a partial and full load PCV separator due to the boost.
The risk is that you're creating a restriction in your PCV system which will reduce the range of PCV operation (i.e. near WOT, when vacuum is lowest). And if the catch cans freeze in extreme cold, the PCV path becomes useless and leaves your fresh air line to take up PCV operation. And if that comparatively tiny tube freezes too, now you've just blown out your engine's main crank seal. If you really want it, fiber type catch cans (using nozzle + filter media) may work well, if you can keep them warm.
I seriously question the statements about the valves fouling up due to low speed service. More than likely, it's due to oil being sucked into the cylinders via the PCV valve.
If you use quality fuel from major brand gas stations, the fuel should keep your valves clean no matter how you drive. The majors all pass the extreme BMW cleanliness test.
I'd shift at those lower speeds mentioned in the owner's manual. You don't need the old 15, 25, 35, 45, 55 mph speeds that I've used for years on all our cars up to the Mazda3 sGT. It is so utterly smooth and strong that you can shift it at far lower speeds or at the 1750 rpm value I mentioned.
Theoretically, running hotter or leaner can reduce valve deposits. The only thing is that the Skyactiv engine has intake variable cam phaser. So the engine is kind of already doing that work for you (still, try to run the engine until it's fully warm up at least).
You should just shift whenever you need to shift. High or low, don't be stingy with the engine power =P