Substantial? Not sure...
Difference? Yes, there has to be...
Lets get down to basics...
Component video out is an analog signal. It's carried over cables and as such is subject to "noise" (of varying degrees), depending upon cable length, cable impedance, the impedance mismatch of the input to the set... and a whole load of other factors. :sleeo
DVi is Digital all the way (if you use a true DVD-D cable and not the analog signal that's carried on a DVD-I cable). Either the signal is there, or it's not. no noise, no signal degradations.. :clap
Now consider this..
Dish is a fully MPEG-2 based system (digitized and sent as digital from the source). If you use component connections, then you introduce noise when creating the analog signal inside the receiver. Then you pick up extra noise in the cables to you set. Then you add noise in the set itself (especially if you run the input interlaced and the set digitises and makes a progressive picture). Result... you've added noise all the way. Could be a little, could be more? Who knows.. too many factors to consider.
Ok, what about DVI.
Well, the "signal" out the receiver is still digital, even into the back of the set, so the only place you could add noise, is in the set itself.
True, a Direct View set is by it's nature analog (electon guns and focusing screens and all that stuff is NOT digital), but I guess that if you're lucky enough to own either an LCD, Plasma or micromirror (TI's DLP technology) then you've got a "digital" picture generation mechansim.
However, I'll stick with my nice new 34XBR910 set.
The bottom line is that picture "quality" is always purely viewer subjective, or interpretive. We, as humans, don't have "perfect vision" anyway!
Hope that helps.