I would target getting the additional subtests administered within a month or two of the original ones, to maximize the chances of staying in the same age norm bracket. If you can't do that, being in the adjacent bracket (most tests are three or four months per bracket) will probably still be more or less okay. The only problem is that GT kids often go through little bursts, so the results might reflect either side of a burst in the same academic area. Another option would be to wait until it's been six months since the original achievement testing, and do the whole WIAT again. The acceptable retest interval for achievement is six months.
And I think it would be a good idea to check on the derivation of the FSIQ. If it is real, then great, and you'll feel more at ease about it. If not, then obviously you need to know that, too. I don't have a WPPSI-III manual, so unfortunately I can't check it myself.
