"Honestly im still waiting for them to release a CPU GPU AIO combo cooler that cools both the CPU and GPU with a 280 radiator in a single loop similar to the Asetek 760GC. Ill happily pay $199 for that "
It already exists and its sold by Swiftech. And it does have a copper radiator so not only better performance, but also no galvanic corrosion driven by the use of dissimilar metals in the cooling system. Not only does it outperform every CLC made (even 360mm units), ... topping the H100i by 7C .... but it does it being > 20 dbA quieter (that's less than 1/8th as loud. Being able to open the loop and add MoBo Blocks, RAM Blocks, GFX card blocks makes it the proverbial "no brainer"
http://www.swiftech.com/H240-X.aspx
There is simply no 120/140mm CLC that can compete thermally or acoustically with a decent air cooler. The 240/280mm can top the good air coolers by a bit but the best air coolers like the Cryorig / DH-15 still rule the roost
http://www.hitechlegion.com/reviews/cooling/liquid/42047-swiftech-h240-x-open-loop-280mm-cpu-cooler-review?showall=&start=3
As for fan swapping..... I have tested this effort extensively using various fans. I was surprised to observe that I lost 2.4 C swapping the Phanteks SP140s (1200 rpm) for the Noctuas AF-15's (1500 rpm). Other fans that I tried showed less temp difference, but for the most part (granted I used fans generally considered among the bast on the market) there were very small differences when rpm was close.
When rpm changed, the effect on cooling was pretty much close to proportional to the rpm change .... a result shown repeatedly in rad testing on the martinsliquidlab.org site. I got pretty darn close to 2/3 the cooling taking a fan from 1800 rpm to 1200 rpm.
If you replace the 100i's 2700 rpm fans w/ 1800 rpm fans then while abetter quality fan may eat up a small portion of the difference, you can expect pretty close to 2/3 the cooling. I suspect that the aluminum radiator is also a limiting factor here as I expect you'd get better performance out of those 2700 rpm fans at the higher rpms with a cooper rad. The impact of the rads ability to transfer heat I would think would be noticeably higher at high rpms than lower rpms.
I used 4 thermal sensors in the loop, one on the inlet and one on the outlet of each of the two rads (420 + 280) in the system (4770k @ 46 multilie / 46 cache multiplier) and twin 780's w/ 26% GPU and 22 % memory OCs) and two air temperature sensors one for ambient and one for internal case temperature to conduct the tests