I'm not sure of the exact implications, but what I do know is that SCI is required to work in certain agencies or areas that handle restricted technologies and methods, such as the NSA with SIGINT:

"For access to information at a particular classification level, individuals must have been granted access by the sponsoring government organization at that or a higher classification level, and have a need-to-know the information. The government also supports access to sensitive compartmented information (SCI) and special access programs (SAPs) in which access is determined by need-to-know. These accesses require increased investigative requirements before access is granted" (Wikipedia)

So if you don't get SCI, any career within Federal intel community would be stunted.

So, I currently hold a Top Secret clearance and I'm going for a full scope/ci polygraph soon. I have done a lot of illegal downloading over the past decade - 10+ terabytes worth of music, videos, porn, applications, and games and I'm still continue to download that stuff pretty much on a daily basis. I know it's wrong but when I'm asking if I have ever committed a serious crime during the polygraph I'm clearing going to be thinking about this. So my questions are:

1.) When answer yes to if that question during the polygraph and have to explain my downloading habit what do you think my chances are of getting SCI? (My record is completely clean otherwise)
2.) Will I be stripped of my current TS if I fail receiving a SCI?