so far not too much, although I am adding the weather stripping around the tailgate which should get the blowback water when driving. I did play with adjustment of the side rails and it seems to seal up pretty well
Yes I love that type of setup and it seems that if the proper weather stripping was used and installed carefully, it would eliminate any and all water intrusion because I don't want anything in my bed getting wet.
That’s why I currently run the Elite Undercover Tonneau cover because I get zero water intrusion but I plan on eventually setting my truck up for overlanding and if I decide not to pull a trailer then I'll have to go with a similar setup.
It just seems in this day and age that the manufacturers could do a better design when it comes to stopping all water intrusion.