I think there is a significant difference in distance between 975D and 975J (even for on-center hits). However, subsequent versions of Titleist drivers (e.g. 983K and 905T) are very similar...except for forgiveness as people have already mentioned.
i totally agree with some of the comments. i recently hit a callaway great big bertha in new condition against an r7 and i hit them the same distance on center hits but when i missed the callaway it was alot shorter i couldnt beleive it because i remeber the callaway being the easiest club to hit back then. that is why the game is how it is today because the guys who are powerful can bomb away and still hit it good in the old days you couldnt do that ball striking mattered more.
Sweet spot is king for sure. These bigger heads are designed not for great players but for those of us out there who need a little help 'around the sweet spot' to minimise the miss of that tiny ggo long spot.