I don't think its too much overlap, but you are right, at some point narrow LSA and duration puts overlap at a point that at low rpm things are pretty "wet" at the sensor and you can do some things to make it a bit less fussy. These aren't in order, I typed as I thought up things that I did to tune my SEFI more to a point that it's pretty much new car like now
Look at FAST's directions or call them to see what they say, but one thing to consider is O2 sensor placement. With normal long tube headers, usually the cleanest place for the O2 sensor is the collector or the reducer. However, this can be too far back and at low RPM or idle the data from the O2 sensor is slightly delayed and does correlate to the specific event that the computer THINKS its talking about
It's more of an issue on SEFI installs, but its always better to put in in the right place as software assumes a certain time between a cylinder firing and how it affects the O2 sensor. If you put it farther back, unless you have a way to adjust for the lag, idle and low rpm can suffer (a little, this isn't critical "wont run kind of stuff, it really depends on exhaust flow, pipe size, etc)
The Powerjection, EEC-IV, and a GM based hodgepodge I did a long time ago, all liked the O2 sensor less than 18 inches away from the exhaust port (more or less), with some narrow band installs recommending as close as 9 inches from the exhaust valve. That usually puts it in a primary tube though, so in the end, just try to get it where the manufacturer wants it, or as far forward as possible.
I actually put mine way back, opposite of what I am telly you here, but in the EEC-IV setup I use, I can manage O2 sensor delay and bias. It still would be better farther forward, but the software gets it 99% there
Another thing to do is really take time to understand how an idle air control valve works on your system, then adjust correctly. They are all a little different, but when adjusted right, the computer doesn't need to make drastic changes at lat idle or transition. There are two things to think about, mechanical adjustment of throttle plates and ECM control of the IAC valve. When you do it right, the IAC will adjust whatever you need it to, if you muscle through and force it to try do more, or ask it to do less than its supposed to, idle will suffer
Finally, if you have the ability to adjust your injector timing, I can help you with a table that will tell you the best time to fire the injector based on your camshaft events, desired duty cycle, target a/f mixture and a few other inputs that I can't remember, let me know.
I did adjust mine significantly. However, note that it doesn't do much above about 2500 rpm as the injectors tend to be semi-continuous and sort of "fog" the plenum, it does however make a difference at low end and idle, essentially making sure the injector doesn't fire during the higher lift overlap periods. Once you get off idle, you could switch injector wires around and not even notice it, even in a SEFI system, but at idle and low rpm, it can make a big difference
