Thanks for Your appreciation, mate.
I can replay to your questions:
1) Dat files ( Track files, without the graphic wad files ) are a simpler way to update a lot of physical data in tracks, instead to use magic files, through the magic editor, as in my previous performances packs. Therefore, new Dat files ( that are anyway based on the original track files, and so on those by Lampert ) are not changing anything on the graphic updates of tracks, including camera views, pit order, etc.
2) dat files included in Erik's update have to be used with my performances text files, and the new Physic file, also included in this fix 2 update.
This , of course, is not mandatory...but is strongly suggested. In fact, it should be a "non sense" thing to use original Dat files, with all other perf files from this new Erik's update.
Indeed, You can use also separately these files, but performances ( in terms of laptimes, driving sensation, etc ) are available only using all files togheter. Of course, one may use the new physic file in any other version of GP4, including those with original Dat ( track ) files...this is a choice.
3) always regarding magic data, and therefore in this case Dat files( updated with my new magic files ): they are essential in updating, in each track, default car set up; braking balance; downforce coefficient; track grip; tyre wear for human driver; tyre type; bhp power for cars in that track; grip factor for CPU drivers; weather conditions; fuel quantity and consumption for all cars; pit stops data; damage data for tthe race in that track; bumping factors for only human drivers; etc, etc.
As You can see, there are a lot of new factors in updated Dat files, respect those original ones, or also those by lampert.