Plug the fans directly into the existing header , there's is no problem or performance hit doing this, I have run Artic and Noctua fans without a problem, I also primarily sim race, with an undervolted profile, but in Benchmarking there is no hit if you run different fans, and probably a gain because of better fans

Your best bet is adding to the stock fans for push pull on the rad, then gaffing taping them sealed around the parimeter.  Should be a 5-9C improvement in temps at comparable RPM.  The extra fans can be run off anything you can dream up, but six fans on the card header will likely exceed 1A which is not recommended.  Best bet is a PWM powered hub ($10 amazon) with the PWM from the card.  MB is fine too. 
 
https://www.amazon.com/dp/B08BFJZWGM
 
Noctua A12's have the best ratio of pressure flow to noise but are expensive, Gentle Typhoons are good but nosier, ML120's are good but loud, I like 120x38mm San Ace server fans.

Not overclocking, so I'd say get something that is reliable at 1500+ RPM.  I'm assuming you are wanting something that is more aesthetically pleasing.

BadTrade
Anyone have a guess at what the Static pressure is for the stock fans.   I was looking at something like Noctua NF-P12 Redux which has 2.83 mmh20 static pressure. I dont really care about aesthetics all that much In my application my only case exhaust will be through that rad and I want to make sure I keep decent cooling performance to sound ratio as wife games in same room.   I may even run push pull.  I just dont know much about the included fans as far as performance so was just going to get some upgraded fans before hand just to be safe.
 
Thanks,  

The only evidence I have is anecdotal , My hand over the radiator. I never used the stock fans on the KPE but I did have a Hybrid FTW3 , Those fans were not horribly loud and worked well, I have not idea why they used the fans they did on the KPE , the hybrid fans I found worked very well on the 240 rad.
I just replaced them off the testimony of others , I now wonder if they were so loud because of the speed , bottom line more rpm can move more air into my hand , only reason I pulled the Artic fans but they are the most efficient and quite.  You could run these all the time but they are limited by rpm and if I got everything heat soaked they are already blowing what they can do and that last 300 rpm cant really pull down temps any faster , the Notua at 2k , I can feel more air but still not as much as those hybrid fans (2500 if I remember right) , I don't know why they were not used on the KPE with all the ****ing about the stock ones. 
 
ANYONE KNOW the RPM of the stock KPE fans?