[Christ]

I don't think the MAP will actually see a difference in manifold vacuum.

Here's why:

The EGR isn't a forced gas, it's drawn in by vacuum. The ECM will see an increase in vacuum via the MAP sensor, but not understand that some percentage of that void is filled by EGR reflow. The vacuum in the manifold will remain the same regardless of where the flow comes from, but the MAP won't be able to compensate for the EGR flow.

That's why vacuum leaks cause so many problems. The engine vacuum is still the same at the valve's face, but the sensor (or carburetor) doesn't "see" the difference.

If you're already running a HAI, I wouldn't think the percentage of EGR you're going to reflow (less than 20% of total volume, I'd imagine?) is going to really affect temps that much, especially considering how much the gasses have already expanded and cooled by the time you get them back into the engine, so there shouldn't really be a need to compensate for temp, but you will need to adjust timing, like Tas said, to make up for the decrease in flame speed.

How much EGR are you planning to flow? That will help the discussion a little.