Chris:
I agree with your assertion that errors in RA during a slew are often the result of not allowing for the continuing RA movement (15" / sec) during a slew. In a recent post on the AP group it was noted that the new AP controller (CP4) now "continues to track" while doing the slew making the slews more accurate. If a scope takes 10 seconds to slew to the specified target point, the sky will have moved 150 arc seconds west during that slew. So even if the telescope's gearing were perfect the point where the scope stopped slewing would be 150 arc seconds off target. This error can be significant (for imaging) when a scope only slews at 3 degrees / second.
It is possible that some telescope controllers take "time to slew" into account when calculating the amount to slew but I have never seen that documented anywhere.
It is sort of like skeet shooting -- you have to aim where the target will be; not where it was.
Charlie