More TIU channels has some drawbacks. One of the biggest flaws in DCS is that it operates serially. So when you add an engine or do a read it does something like this"
for (TIU=1;TIU<5;TIU++){for (channel=1;channel<4;channel++){LOOK FOR TRAIN!}}
Meaning it goes TIU by TIU channel by channel 1-by-1 to add engines, read, or anything else. Nothing is done in parallel. Adding engines takes literally 3-5 min with our 5-TIU setup (compared with 20 seconds on my single TIU layout). If you use the wifi APP things get frustrating after 2 TIUs. It just takes tooooo long to read, find engines, and stuff. Often the app will give a TIU timeout if all the checks don't finish in time.
