I don't agree with them. I feel a post filter is a waste.
The factory filter filters @ 7microns. The whole fuel rail system and injectors along with the factory filter is built by Denso.
So what Denso are saying is the system will tolerate any crap below 7 microns.
What are you going to do?. Fit a 2-5 micron filter after the factory one? Why, just to block it up with crap that the system can deal with?
Once that 2-5 micron filter starts to do its job, I'm guessing the fuel pressure will build considerably. Yes i've seen their video, but thats with clean fresh fuel in a controlled environment. Not what happens in the real world.
A 20-30 micron pre filter is more than enough.
Personally, i'd go the water watch way. Let the factory filter, filter crud only and have the water watch filter take care of water.