When I was struggling with the Champion fans referenced above, I was ready to pull their whole shroud/fans out and replace it with one good Spal fan. They are a bit pricey but they move a TON of air.
I talked at length with the Champion Cooling solutions rep about Spal. They have CFM down low and up high. We discussed the Spal 30102113 that was 257 each and moved 2467 cfm. That's was my choice if I went with Champion CS. With the 16" fan from Cold Case pushing 2800 plus change. so I feel confident in that department. Of course there is higher CFM fans from Spal and I may have to look into that if this 16" fails the test. It was funny but all the reps said single fan over duals. interesting