I don't use smartcast or quickcast or whatevercast so I'll see about barelling it up in a custom match a for a few minutes and see if it strikes me as delayed. It might be a really weird interaction with the other casting mode or something based on what little I see in the thread so far.
Edit: After ten minutes of testing with normal cast for about 8 minutes and quick for 2 I can say that I didn't see anything that would seem out of the ordinary but I'm not really a big GP player. I went so far as to flail the mouse about randomly while casting the barrel and it always went where I clicked. As for the quick-cast portion of the test.. well I've been playing this game for over 6 years and literally never used QC before so it just felt wrong in general so I can't really help with that.
If the difference in delay is really under a split second its conceivable I just wouldn't notice it as a non-GP main. Sorry I couldn't offer more for you.