It has been pointed out that there are differences when comparing 450d to 450b High Gain. While the spec sheet seems to indicate only a 1 dB difference in gain (24 dBi vs 25 dBi), many are seeing 2 or 3 dB difference in practice.
The graph below illustrates why.
As noted, the 450b ranges between 1 and 3 dBi lower in gain throughout both the 5.4 and 5.7 GHz frequency bands. The peak gain for the 450b High Gain dish is accurately specified at 24 dBi, while the 450d was also accurately specified at 25 dBi.
Another thing to note:
If you are comparing the Receive Power at the AP between a 450 SM with a reflector dish attached and the 450b High Gain module, you need to ensure that you have the correct “External Gain” set for the 450 SM, or you may be exceeding the EIRP limits in bands that are limited (eg. 5.4 GHz in FCC).