Spoke too soon. I can't seem to make much sense of it.
The values I change the Y Speed of the Warp Effect to seem to ignore common sense, and have rules unto themselves. Check out this quick .CAP I put together to show the issue:
The wave starts out with a Y Speed of 5.
Left Arrow sets it to -50
Right arrow to +50
Up Arrow to 100
Down Arrow to 1
A sets it to 60 (so you'd expect it to be slightly faster or slower than 50)
S sets it to 40 (again, slightly slower or faster than 50)
W should set the value to 5 again.
The results however... are bizarre. It's like each value has its own speed, completely irrelevant to the speeds of similar values. So while 50 is a moderate speed, 40 and 60 are batsh*t insane. And setting it to 5 using the W key is a completely different speed to the default 5.
Maybe I'm missing something? Either I'm using it wrong, or the action isn't working as intended.