That's kind of strange. 1/2id in both cases. I wonder if the compression fittings were pinching and restricting the hose somehow. That's the only difference between the two I can see.
You don't have a flow meter?
And yes, mods can be a nightmare if you install carelessly or don't pay attention to what you are doing.
As a general rule of thumb, I don't like to install mods manually. Too easy to screw things up that way and troublesome if you need to remove or reinstall. I prefer to use mod managers whenever possible. Especially when you're chasing down a conflict.