user-1: I am using a 32u4 on a custom PCB: 3.3v, 8MHz.  
Are there any operational differences between using the Sparkfun ProMicro selection versus the Adafruit ItsyBitsy selection in IDE, since they are both for the 32u4?

user51836: There should not be any difference, However you would need to load the ItsyBitsy bootloader instead of the ProMicro Bootloader. Personally, I use the Pro Micro Bootloader on my 32u4 Boards.
The only difference would be in the size of each bootloader, one may be smaller and give you more actual codespace. Other than that there should be no performance difference because your code runs independently of the bootloader.

-1: <p>I am using a 32u4 on a custom PCB: 3.3v, 8MHz.  </p>

<p>Are there any operational differences between using the Sparkfun ProMicro selection versus the Adafruit ItsyBitsy selection in IDE, since they are both for the 32u4?</p>
 51836: <p>There should not be any difference, However you would need to load the ItsyBitsy bootloader instead of the ProMicro Bootloader. Personally, I use the Pro Micro Bootloader on my 32u4 Boards.</p>

<p>The only difference would be in the size of each bootloader, one may be smaller and give you more actual codespace. Other than that there should be no performance difference because your code runs independently of the bootloader.</p>
