Some of the original tapes were leaked on the internet in 2007, I saw footage before. It was nothing. The guys being filmed even WAVED BACK at the amateur cameraman. Videotaping sideline hand signals is STILL LEGAL even now in a dozen spots in the stadium, just not from the opposing sidelines.
"Spying" is a completely exaggerated term as well, given that hand signals are publicly viewable by everyone in the stadium, and also the TV audience.
I am completely convinced Goodell destroyed the tapes, because revealing them would show how biased and un-objective his punishment was. I also remember the Patriots, Kraft, Belichick were barred from saying anything after the tapes were destroyed, giving them zero chance of telling their side of the story.