I don't see have the flex is going to make much of a difference. The fan's impact is at low or no speed where the flex and non-flex should be equal. And both fans are designed to zero themselves out at highways speeds where yours is getting hot again.
Looking at your temps vs speed info, I'm thinking you just don't have enough water flow. Is the WP pulley the correct size? Does the Flow Kooler water pump come with the cast impeller vs the stamped design that many have found to make a big difference when looking at replacement water pumps?
The distilled water/water wetter issue is unclear as to whether you put in any coolant/antifreeze. I run 50/50 distilled water/Prestone. I mix it myself vs the pre-mixed versions.
If you car is going over 35mpg the air passing thru the rad is enough to cool the coolant. If the car get's hot at higher speeds then it cannot be a fan issue.
What you need is Temp sensors in difference places to determine where the bottle neck is. One before inlet to rad, one on outlet of rad etc... Not sure how you can do that,
Maybe you have some flow restrictions in your cooling system like someone else mentioned.
DO you have the filler panels installed and the lower pan? That will ensure maximum air flow thru the rad.
