I'm building a new computer in a few weeks whose parts I have gradually accumulated over an agonizingly patient half year. Its cooling is exclusively Noctua, which I expect will look great in my green Corsair C70 Vengeance case: 5 (possibly 6) NF-S12A FLX and 4 NF-A14 FLX case ventilation fans and even one of their 40mm fans as a tiny upgrade to the ASUS Sabertooth Z77 motherboard's thermal armor feature (the other fan is only around 35mm, as I understand it, and cannot be so upgraded, sadly).
For the CPU, I'm using Noctua's NH-C14 heatsink/cooler, which I selected primarily because the downward airflow is recommended with that Sabertooth motherboard. Now, I'm sure that the fans it comes with are perfectly fine, but if we can't get silly about our hobbies then what's the point? I want to make it even better by using the NF-A15 PWM and NF-A14 PWM, the latter of which should begin to show up in stores in a week or two.
Here's the rub: I would just use two of the A15s, but Noctua show that one would have to be slightly offset from the heatsink in order to fit, as shown here (scroll down a bit for the C14). That offset bothers me. Now, I spoke with Noctua and was assured that it would still represent a performance upgrade, but I want to avoid it anyway; if a thing just doesn't fit right, it will never sit right in the back of my mind. So what I want to do is to use the NF-A15 PWM as the top fan for the NH-C14 and install the new NF-A14 PWM on the bottom, which won't require an offset. What concerns me is that the new A14 PWM has a higher RPM of 1500 versus the A15's 1200 as well as an airflow difference of 140.2 m³/h versus 115.5 m³/h.
I did a lot of research and read conflicting opinions on whether different fan speeds in push/pull configurations could cause turbulence, unwanted wind vortex patterns, decreased performance, or even dangerous mismatched resonances. The PC enthusiast community at large just seems to be uncertain of the science of these air dynamics. Noctua wouldn't give me a straight answer on the matter because they had never tested that configuration (unsurprising since one fan isn't even in stores yet).
One of the features of the Sabertooth motherboard is fan profile configuration where you draw fan speed graphs to correspond to temperature levels so that they don't just run full tilt all the time, but I won't know what this is like to use until I get in there and I just don't know whether you can control the two CPU fans independently, which is what I'd need to do if I wanted to keep their speeds matched. I could probably just attach Noctua's low-noise adapter to limit the voltage of the A14 PWM and bring its RPM down to 1200 as well, but I don't want to do that unless I have to.
So what do you all think about this? Is it a bad idea to allow the two fans to run on either side of that heatsink at these different speeds? Is it dangerous, or would it simply lead to decreased performance? Would I be better off limiting the faster fan? Would I be better off doing something different entirely? I'd appreciate any guidance you can offer, and I'd also be interested in any theoretical conversation on the subject!