As long as the premise is "well i have tried 4 interfaces on 3 different computers, and the issue is still there, wish they write a new algorithm just for this issue" it will
never be possible to find the source of the problem
because it has nothing to do with the input echo algorithm. An input echo button simply passes audio through the computer to the output. As Larry Jones has pointed out, thousands of musicians are using input echo with SONAR to monitor with low latency through effects. I am one of those thousands. If you are getting extreme latency, the issue is either how the system is set up, or the operator. These are beyond the control of the manufacturers who make the gear we use.
If you're using Thunderbolt, who knows what's going on with your system - very few PCs are Thunderbolt-friendly, and you need a very specific chip set. Also, 6 GB RAM is hardly enough for Windows to even wake up in the morning.
As to monitoring through reverb, the effect itself typically has more milliseconds of pre-delay than most people ever experience with an interface. As detailed in this thread, there are other ways to record with reverb in the voice. Given the basic attributes of an Apollo interface, there should be
no detectable latency problems recording vocals while monitoring through reverb. The only latency is due to A/D and D/A conversion, which is unavoidable with digital gear and is less than 2 ms.
I don't want to come across as snarky, because I know how difficult it is to get Windows-based systems operating properly, and I sympathize. Regardless, when people look in the wrong place for a solution, they will not find a solution. Somewhere in the system - and it could be a SONAR preference - something is not set correctly.