Have you removed those fan grills that Lian-Li uses already? They are very restrictive, and make the fans noisy. Also, what are the temps? In my experience, a few of those Nexus fans create almost a wind tunnel. If you stick a temperature sensor (or your hand) in the case, does it feel hot or does it feel like ambient?
Also, those Nexus fans are pretty good compared to Noctua fans, according to this review at Xbit-labs:http://www.xbitlabs.com/articles/cooler ... html#sect0
They've made a graph of something interesting as well:
Very impressive and illustrative difference, isn’t it? And note that all fans were working at the same level of noise, in other words, by choosing a proper fan you will be able to significantly lower the CPU temperature without losing any of the acoustic comfort. If we go back to the airflow readings, we will recall that Akasa fan generated about 34.2% higher airflow than Noctua at the same level of noise. And as you can see from the obtained results, it greatly affects the CPU temperature. The airflow from Noiseblocker fan, in its turn, is only 18.8% higher than that from Akasa, which translates into smaller CPU temperature difference. In other words, we see evident practical continuity of the results taken with our thermal anemometer. I would like to add that Nexus Basic fan that is about 4% behind Noiseblocker in airflow taken at the same noise level lost only 1 °C (when tested with two fans on a heatsink):
So, the Nexus working at the same noise level as the Noctua gives a temperature 8 degrees lower than the Noctua
If you replace a Nexus with a Noctua, you might have to let it run at +200RPM for the same airflow, meaning that you won't gain much airflow by replacing your fans.