@Quagmire - thanks for the tip about reverting to stock torpedo file
Last time out I fired off all 20 torps and got no hits. It bummed me out more than I thought at the time, and reading about all these modded torps (and arguably stock torps) veering off course wasn't improving my outlook, so I had taken a few days off to wait for RFB 1.28 and/or the patch before heading back out on patrol, but then I saw your post. I decided to swap in the stock torpedo file as you suggested (didn't have to uninstall RFB - just created a new mod with just the stock torpedo file in it and installed it over top of RFB with JSGME).
Spotted a lone Japanese medium freighter soon after loading my saved game (mid patrol - saved game from a bit before I shot off all 20 torps last time - just wanted to see what would happen mid patrol). Despite a perfect PK setup in pretty calm seas (maybe 4m/s) at 1,000 yards, I missed with 2 of the first 3 torps I fired - not sure what happened to them, but given the time (late 1941) and Mk 14s, I figured a 33% success rate was pretty reasonable. The one that hit, just abaft the bridge, brought her to a standstill and she flooded somewhat but did not seem to be sinking further, so I decided to turn around and finish her off with one from my stern tubes. From 800 yards, my 4th torp ran hot, straight and normal and struck her amidships, and she went down a couple of minutes later.
So, overall a 50% failure rate, but also a feeling that at least they'll work some of the time. A much more satisfying start to this patrol. Of course, this could all just be random chance and have nothing to do with the modded torpedo file in RFB, but an interesting data point nonetheless.