I had used ACS customs and they were horrible in terms of altering the sound. Very bad. Rin posted a measurement of customs as well, and "typically" due to the depth and length of the custom sound tubes, etc. it is nearly impossible to retain the neutral signature with custom molded tips. At least from the multiple brands that were tested.
http://rinchoi.blogspot.com/2013/05/etymotic-research-customfit-ear-sleeves.html?m=1
http://rinchoi.blogspot.com/2010/11/customfit-ear-sleeves-for-er-4.html?m=1
I even sent mine back mutliple times, and the tried to get the er4 housing deeper into the molding and work with me, but it didn't help help enough. And placing filters in the molded eartip was suggested but also changes the response noticeably. In essence, for comfort, maybe. For sound, they're awful.
Now I don't know if some other method of custom tip would be better, like going through westone or something...
http://rinchoi.blogspot.com/2010/05/how-deep-do-you-insert-you-er4.html?m=1