Personally, I would not skip them. While the GAI is useful, I think having the entire picture is best. We have kids here w/ very high GAIs but with avg, high avg PSI and/or WMI and these kids remarkably are quite similar in some areas. I do think those subtests impact learning. For ex: a friend's DD has a PSI of 150. The psych said she had never seen a PSI so high and that it explains why this DD is phenomenal on the soccer field b/c she processes information so quickly and is able to act on it immediately. I think a kid w/ FSIQ=GAI of 149 may look different from a kid w/ FSIQ=134 and GAI of 149.
This is one area I wouldn't skimp on the money.
I would pick up Developing Math Talent. The SCAT is a nice quick test and you get results back quickly. It's not an achievement test. For achievement, Woodcock Johnson III will give you math scores. For grade placement, I think you need to go to taking out level achievement tests for example, the school's own end of the tests or something like the IOWA Test of Basic Skills 2+ grade levels up.
