You have to remember that the game we all play is Race 07. All the rest are just expansion packs which add new content to the original game. It doesn't matter what the name of the expansion pack is. Whether it's GT Power or Injection is irrelevant... both add the BMW M3 GT2 to game. Whether it's STCC 2 or Injection doesn't matter... both add the Chevy Cruze to the game.
If you check the game's info page at RaceRoom, it tells you exactly what cars it includes and which expansion they were originally from. So, if an online event is using one of the cars on that list of from that expansion, you can join. If it is using a car which is not on the list, you can't. Simple.
As for the release itself, I honestly don't see what the fuss is all about. What is wrong with SimBin releasing a final version of a sim which has been very successful over the years? As it is all existing content it took them minimal time to produce, if any since Namco may have packaged it up rather than SimBin.
Given the fact that SimBin have been reported as struggling, do people really begrudge them trying to make a bit of extra cash from completely new customers? It's not like it's another minimal expansion aimed at us, is it? And which would people rather, a final release that tides SimBin over until GTR 3, or SimBin going out of business and never getting to release GTR 3 at all? Think, people.
My only gripe about this is that GTRE and RO content are missing from the Injection. I don't see the point in that. It should have been a final be-all-and-end-all release of the entire R07 package.