It certainly CAN be lack of exposure and perfectionism - but where does the perfectionism come from? Often from the child perceiving the gap between what they "should" be able to do and what they can achieve. Sometimes kids have overinflated ideas of what they "should" be able to do, and their perfectionism is based on unrealistic ideas - sometimes the gap is due to the fact that they really are that smart but can't focus, or can't control their hands etc.
The combination of much lower processing speed and the big achievement gap and the difference in behaviour certainly suggests it's worth fully exploring.
Also keep in mind that it is VERY difficult when reversals, etc ARE age appropriate (for a normally developing child) to figure out whether this is an area of relative weakness that will be quickly overcome or whether it's a sign of something significantly out of step with a very gifted child's development and needs to be heeded as not "are appropriate" for that child.
Yes lots of kids struggle with handwriting and with reversals - but should a kid with a FSIQ that is nearly 4SD from the mean struggle with them more than fleetingly (especially reversals)?
