It completely isn't. "As a package" doesn't absolve you of claiming it's widely regarded as the best engine - you are basically implying it's the one to have over everything else, and it really isn't if you can afford the myriad of better engines than the 3.0 TDI.
Buying an A5? Want the 4.2 V8? Nah, get the 3.0 TDI because it's the best package! The reality is the 4.2 V8 is much better in every single way except cost.
And if they did the v8 on the a5 you would have a point, but they don't. They don't do a v10 on the a6 either.... Not I didn't mention any S or RS models as it's pointless including them when discussing the common models.
So bringing this back to the q7 would you really recommend someone gets a petrol one given what the OP has said thus far? Really? You wouldn't because it would be stupid.