Blowback buffers for the AR are a little longer to compensate for the greater travel possible when firing the shorter cartridge. Standard buffers require a spacer.

Does the MGI buffer need a spacer when shooting 9mm? If not need, should it?

The MGI buffer is about 1/4” longer when fully extended.  My guess is you still need the spacer.

Yes, you should still use a spacer if you want to use the MGI buffer with a straight blowback setup.
BTW, below is a picture of one of my MGI buffers where the spring broke.  Still worked fine and I don't think it really does much.
When compressed it is the same length as a standard carbine buffer and it only takes a small amount of force to compress.....so just get a spacer.  I like the New Frontier Armory spacers.

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Thanks for the answers, guys! I recently acquired a mint one for my Hydra and am looking forward to testing it out for all calibers. MGI's website recommends using the RRB for 9mm, but did not mention anywhere about using a spacer and kinda implied it wasn't necessary due to the RRB's design. The thought was nagging at the back of my mind though.

Thanks for the confirmation and recommendation!