Problems with visual processing and visual-motor integration could certainly have an impact on the subtest scores used to compute the PRI and the PSI.

My DS, who is not dyslexic, but who is dysgraphic and has AS/NVLD, and who has problems with visual processing and visual-motor integration, scores like this on IQ tests:

http://giftedissues.davidsongifted....e_WIAT_III_subtest_scores.html#Post94041

The RIAS does not have reading or motor output requirements, and the visual items are easier to see than those on the WISC. The RIAS also doesn't include social comprehension items (a problem for AS/NVLD individuals)in the verbal index score. The differences between the WISC scores and the RIAS scores here can give you some idea of how these kinds of deficits can impact scores on the WISC.