We tested our DD on the WPPSI at 4yrs9mths and had multiple ceiling issues, we then tested at 5yrs2mths with the SB5 and her FSIQ was 8 points higher. To my mind the three big reasons were
a) gifted specialist tester who had far better rapport with her
b) test fatigue on the WPPSI, even on the SB5 I could hear she was getting tetchy at the end and it took half the time.
c) ceiling issues on the WPPSI, she hit one hard and multiple soft ceilings on the WPPSI, but her highest scores were 17s, sometimes due to scaling (she was too old to score better than 17 on one subtest) and partly due to getting lots of 1 out of 2 scores because she didn't like talking to the tester.
I personally would not recommend the WPPSI as a tool to identify a gifted 5+yr old. The risk of needing to retest them very soon is too high.
