Well for the things I'm doing, adding polys will pretty much always make things look better.
I really doubt that. Maybe yours is the first, but I've never come across a game yet where it was.
Roughly how many triangles do your games/projects tend to end up with after code/LOD optimisation is put in. I know this will vary a great deal but the rough average number is probably a good guideline to use when it comes to deciding how much LOD/optimisation work I need to do. . .
I respectfully disagree. I think this will send you right up the wrong path and you'll either have a game that could have looked tons better or you'll have a gorgeous game that runs like a slug in treacle. You're not making an average game, you're making YOUR game. The only worthwhile way of finding out how many poly's you can push is to try it.
Anyway, you're clearly intent on working from an arbitrary average and I've said all I can to caution you against making a mistake, so I'll leave you to it. I genuinely hope you prove my concerns to have been misplaced.