the more apart will your left and right ears be, the more you might benefit from customs as they can carry the sound with less interference from the in ear canal.
but in practice I'm not even sure I agree with that idea, because if we have an imbalance of sort, in loudness or in frequency response, we have it all the time, so our brain is compensating for it. wouldn't the custom create the opposite imbalance?
well I really don't know, and I guess it depends on how much time you spend with the CIEM ^_^.
another important factor is the distance between the driver and the eardrum. with CIEM if well done the guys should have a pretty good idea of the right distance based on the bends on the molds. but then you can also just learn how to use your IEM and where to place it to get the best response.
obviously that depends on the shape of the IEM, an ER4 can be placed almost anywhere in a 2cm distance and still seal pretty well, so it may take time. something like a shure or westone, you don't have that much of maneuverability. you put it where it goes and you're done, if it's not right, well too bad.
yes the tips you use have an impact on the sound. that much is very very clear. in a way we could see the custom tip as just another tip with another impact on the sound.
objective data, I don't know, but for tips you have many on the M.R.O blog like this one http://rinchoi.blogspot.fr/2012/12/the-effect-of-ear-sleeves-westone-4.html