It very well may help. I'm not certain I can confirm or deny, as I think I have blowby around my cleanout plugs. My cleanout plugs are super easy to put in, never even an inkling of an issue with the rubber sticking like a few others, which leads me to believe I won't be able to confirm the check valve personally, until I rebuild my plugs. Since it's not an issue at this point, I won't be rebuilding them anytime soon, as I'm not really losing any noticeable speed - at least to me. For all the guys on here who can't stand the idea of puttering around at 30 mph, or having their top speed at anything less than 50 mph, this might be a huge difference, but for me, I still get to all the bars and restaurants without complaints.
Now, in 3 weeks when I pull the boat out of storage, Murphy's Law says the cleanout plugs will require rebuilding.
@RobbieO if you can't read every thread on the forum, are you really even following along?
I know someone else on the forums added a check valve, and it worked for them - another point steering me towards the rubber on my cleanout plugs letting water past. If I can find my little knockoff go pro, I can check it out when the water's high enough to drop the boat in. Until then, the responsibility has been passed to you!