Yeah, air saturation levels on a single MCR are already in the 50-70% range for the fans we use (slow to ~3000RPM). Stacking two will theoretically get air saturation levels of 75-91% based on those numbers, which is pretty significant considering how little of a downside there is (no added noise like a higher speed fan, no huge space requirements like adding another rad in parallel, etc). What it ends up being....eh, we'll see

In theory, going to three won't be hugely significant, 87-97% (and again, probably less in practice) air saturation levels. Going to 4-5 will be even worse in terms of gains just because there's less to be had below the 100% ceiling.

And considering higher RPM fans (that would benefit from the more radiators), already are associated with low water to air deltas, the benefit from going from 75% to 87% will be pretty low, unless there's a massive heatload.

I wouldn't worry too much about suffocating fans with 2 or even 3 MCR series. I have an MCR320/MCR220 stack right now and the difference between the open fan and the two buried fans doesn't seem too bad, even at low RPM