Yes, quite surprising to me too. I did expect a change, but not so big. I find this whole story surprising in that specific regard: everything I am doing seems to produce a change 10 times bigger than I expected.

Anyway, to sum up:

I installed current 9cm fan precisely so I could push it further in, closer to the HDDs, and hopefully take advantage of the air intakes at the bottom of the case. This setup is not as good as hoped for, even setting the 9cm fan at its top speed.
Therefore, I am resorting to dremel air intakes. But this means there's no particular reason to use a 9cm fan in that particular position: I'll revert to a larger and slower (and quieter) fan.

Last time I modded a case it was years ago, so I ordered masking tape and other stuff I realized I missed. I'll probably be working on this next week.

In the mean time, I may try some other fan configuration. Actually, I am afraid the only thing I have not tried yet is a large fan with the side panel put back in place. I decided that would not work because the large fan would be right against the metal panel, but, at this point, who knows...