318 Internal Vacuum Leak?

I'll go out on a limb and say that your valves and rings are fine enough to not be the cause of your problems. Your 10 degrees of Idle-timing is fine.
I'm going with the ECU not knowing the Engine temp, or the temp is not getting high enough to trigger the ECU.

With EFI, (as with a carb) assuming the throttles can be closed;
there are only a few things that can cause a High Idle, namely;
1) too much Idle-air, and
2) too much Idle-Timing
3) a faulty warm up program/ECU stuck in open loop/AIS not coming down.

IMO, to set the base settings, one needs to defeat both the Closed-loop system, and the Automatic Idle-Speed motor; which means taking the 02s off line. As long as the ECU knows about the issue, it's just gonna keep adjusting the AFR to get to whatever it has been programmed to do.
As for numbers 1 and 2 above, you probably got that covered.
As for #3, above; Your EFI must have a warm up program. Which is controlled by either Coolant temperature or time, or some combination of those. During this time, the ECU is already in open loop. Perhaps your coolant temp sensor is not working? and the ECU is waiting for the temp to come up. During this time, the ECU is on a base map, but the AIS controlling the Idle speed to a programmed rpm.
>Disconnecting the AIS will not affect the idle speed, once the AIS has jacked it up.
>After you shut the engine off, the ECU is pre-programmed to adjust the AIS to a high idle, so that, on the next cold start, the AIS will already be at a high idle.
>After the ECU figures out that the engine is warming up, it will reduce rpm via the AIS, proportionally to what she is seeing at the CTS.
>If you start it up hot, the ECU is programmed to rapidly bring the rpm down, proportionally, to what the CTS is telling the ECU.
>At the moment when you shut the engine off, the ECU is programmed to shut the air off at the AIS, to prevent run-on. This means that the Curb-Idle Screw has to be set closed enough, to not interfere.

All of this assumes that the ECU knows what the Coolant Temp is. Therefore, I would prove that device, and it's circuit, and/or if you can electronically get into the Sniper ECU, like with a scanner, I would check to see what the ECU is reading.
Good luck