Postponed my impressions by a day, seeing as I had an overextracted espresso yesterday due to the fact that I ground the coffee too finely. This morning, however, was a far different story: a perfectly calibrated shot, pulling 2oz in 26 seconds.
The result in the cup? Best espresso I've had to date with my Gaggia Classic! Admittedly, I am using a new batch of beans... but my espresso was far less bitter than what I was getting before, and I can now taste a greater range of flavours.
Bottom line, it seems like reducing the pressure to 9 bar was definitely worth it, and I do recommend it to all Gaggia Classic owners.
Thanks to everyone for the helpful tips.