It only benefits, if you have special tasks. E.g. you have one Wireless device that transfers at full speed over wan. Then all wireless interrupts can stay on one core and ethernet interrupts on the other. Good for benchmarking. But in real life you have mutiple devices that transfer via lan,wireless you have cpu intensive tasks that also utilize these core. When both cores are utilized with several things the overhead that is introduced with transfering memory from one "core" to the other (context switches) will cause a negative effect. If you have a modern CPU with 4 or more Cores it is different, there you have enough cores which handle other tasks such a smb processing nat etc. there you want to make sure you have enough cpu power assigned for interrupt handling so you have handle the amount of interrupts, that come from ethernet, camera and other input devices, than you benefit is higher then the penalty that comes from shifting memory.between the caches etc.
If you would do a benchmark, with multiple devices from wan lan to wan and maybe have something a proxy (privoxy) running or smb to an attached drive, then you will definitely see, that the total throughput will be smaller. Otherwise you wouldn't need irqbalance, as it can only balance between 2 cores, you could easily set this your self, by pinning ether on core 0 and wireless on core 1. Which I have done for testing:-)
But as I said, this is only usefull if you want to achieve something special.