Improve changelog

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/ChangeLog.d/aes-perf.txt b/ChangeLog.d/aes-perf.txt
index 6969457..ab716bc 100644
--- a/ChangeLog.d/aes-perf.txt
+++ b/ChangeLog.d/aes-perf.txt
@@ -1,4 +1,7 @@
 Features
    * AES performance improvements. Uplift varies by platform,
-     toolchain, optimisation flags and mode, up to 4.5x.
+     toolchain, optimisation flags and mode.
      Aarch64, gcc -Os and CCM, GCM and XTS benefit the most.
+     On Aarch64, uplift is typically around 20 - 110%.
+     When compiling with gcc -Os on Aarch64, AES-XTS improves
+     by 4.5x.