WMI and PSI are grouped together because one of the rotations on the factor analysis groups them together. That's the simplest explanation. From a functional standpoint, they group together because they both impact efficiency (hence the Cognitive Proficiency Index in the Wechsler family). It is not uncommon for gifted individuals to sacrifice efficiency for maximum accuracy and precision (which often involves mulling over all the possible permutations and angles of a problem).
WMI and PSI both have been associated with ADHD--WMI because it's hard to hold things continuously in your memory if you have dysregulated attention, PSI because of various factors, including impulsive responding (high error rate) and difficulty sustaining attention throughout a somewhat monotonous task. But some kids with ADHD score very high on PSI because they don't know how to put the brakes on. If your impulsive responding is highly accurate, then it becomes kind of an advantage.
Vision issues would definitely differentially affect PSI, as it consists entirely of tasks which require attention to visual detail, often with visual-motor responses. The remaining visual stimuli on the WISC-IV are relatively large, brightly colored, and can generally be recognized from the overall shape. (Unless the supplementary Picture Completion subtest is administered. That is specifically a task of attention to visual detail.)
