Picked up a 3.861 and did a quick test with it right next to my 3.825.
With BIG (L) mode on the performace seemed to be about the same on KA. In one location the 3.861 alerted to a K that the 3.825 did not. Interestingly enough that K detection appeared to cause a rear facing X band detection on the 3.825.
I then did a run with "A" mode on both units, again side by side. Both detected Ka pretty much identically. The bogey counter on the 3.826 seem just a hair more stable, as did the directional arrows. During that Ka detection both radar detectors also alerted to X band coming from the front (KA was from the rear). The X band detection seemed a bit strange. I think I will try a few more tests.