That varies a lot, depending on motherboard and the positioning of the 120mm fan. The 120 can be use for better performance under the right conditions.
I don't have experience using the 120 adapter, so I wish you luck.
I didnt have good experience with 120mm, it was worse than default fan wo duct. Now I have the duct, works much better.
The 120mm hub is very Big ans creates a deadpoint on the heatsink.