Just got my E17 yesterday. Wow. I feel like all this time I haven't been listening to music correctly. The clarify and detail these give me compared to the very mediocre and down right bad sources ive used in the past is night and day. Yet, I know that im only scratching the surface.
Will I notice a different with a "very high" quality interconnect cable compared to the one provided with the E17?
It depends on how you are connecting your E17 to your source. If you're connecting to the E17 via optical or coaxial SPDIF cable or via USB, my opinion is that you won't notice any difference by using a 'high end' interconnect. Digital is digital - it either works or it doesn't.
Now if you're going analog, and I assume you are since the E17 only comes with digital adapters and analog cables, then the better quality interconnect MIGHT make a difference. Personally I can't tell the difference with my ears, but you might have better luck. That said, a good quality analog interconnect won't hurt at all, and likely will last longer than the one supplied by Fiio. I use an Apple iDevice as my source so I tend to use the Qables Silver Series performance LOD to connect to my E17. I've also used the Fiio L9 and L10 LODs, but both of them broke after a couple of months of use. My Qables LOD is still working well after 4 years and shows no sign of wear (I bought it for use with a different amp/DAC, but it now gets used exclusively with my E17).
My suggestion - concentrate more on the source feeding your E17 than the cables connecting them. Finding a good source that is portable that supports playback of many formats and one that has a digital output is still my holy grail... there are some decent media players out there, but so far nothing that does everything I want. Your preferences are likely different so you might have more options. Good luck!