Memory and processing scores that aren't significantly different from VCI and PRI shouldn't lower FSIQ significantly. A difference of as few as 6 points between FSIQ and GAI is considered "significant". That said, GAI is certainly the appropriate composite to use for gifted program admissions. 40% of children identified as G/T had FSIQ 5 points less than GAI. If WMI and PSI are low enough to affect performance, appropriate accommodations may be needed.

These links are relevant:

http://www.ksde.org/Portals/0/Special%20Education%20Services/gifted/WISCIVTechReport4.pdf

http://www.nagc.org/index.aspx?id=2455