When I demo'ed the 335DW, it was relatively bassy with not much treble extension. The custom was much more balanced, though still had that warm character that made me order it in the first place. Based on that one experience, I'd say the demo only give a hint of the custom's flavour, but everything pretty much improves once you get that seal that comes with a made-to-measure unit.
It was a bit of a blind leap on my part, since I felt the 334 demo was more balanced than the 335DW, but in the end I opted for the latter because I liked the body it gave the lower frequencies, particularly with the type of music I often listen to.
Having said that, the demo experience probably varies from person to person. Our ears are all shaped differently, after all. I suppose you could get a demo to sound closer to the custom if you played with different tips and whatnot. FitEars are probably one of the less "risky" demo experiences due to the way they extend into on your ear canals, which is quite shallow compared to most of the other brands.
Edited by kurochin - 7/11/14 at 5:27pm