How much does disabling early_demux actually matter?

It depends on what kind of packet rate one is pushing through the device, what type of hardware is being used as the router and how sensitive ones applications are to such things but if you want a better scientific answer run as many load tests as you can that measure packet rates, lag and jitter then disable them and run your tests again. I should add this is just one tunable related to routing. If bored dig into this a lot deeper and one will find there are many things that can be adjusted at the NIC, in the OS network stack and so much more.

Some people like defaults because it is less cognitive load and some like to tweak things until they can't get better results. I believe that everyone should be able to choose their own path to satisfaction. If the defaults and fq_codel work for you then that is probably the right answer.