The only attempt I have seen to quantify a difference is the two charts at;
However, based on this thread:
and knowing that Noctua and 45HL have an interest in selling the kit, I’m not sure how much I trust those as being objective and scientific. In my experience, the issue with the fans and HL15 noise isn’t the fans, it is the way they are wired. If you run the Noctua fans at 100% like the stock setup, they are going to be loud; if you hook the stock fans up to fan headers that support RPM control like the kit, they will quiet down. The issue with the full build was the Supermicro X11SPH doesn’t support DC fans, and 45HL was too cheap or something to use PWM fans connected to the motherboard. Measuring dB v RPM is sort-of pointless; you need to measure dB v equivalent CFM.
I apologize, but for $900 it grinds my gears a bit that they can’t provide a set of reasonably decent PWM fans as standard; that’s all they need to do, other manufacturers can; there’s no need for a fancy kit, although they seem to be charging a fair price for the kit compared to buying the components separately. ˂/rant˃
Anyway, qualitatively, yes you need some sort of fan mod if you aren’t putting the HL15 in a datacenter. For me it was sufficient to hook the OEM fans to my motherboard, but my HL15 isn’t in the same room as me. Perhaps it would be a little quieter with Noctua fans; my Q30 had Noctua fans (direct from 45Drives as standard!), was in the same room and was pretty quiet.