GT 5.5 vs GT 6.0 is an argument based on how a product feels when playing or otherwise perceived by the end user. Not a literal itemized list. Games aren't summed up on "how much", but more how well you used what you got. Something as easy as new sounds and a livery editor would change the majority perception of GT then updated existing features including rendered and physics as those are core to any product like this anyway.
Games aren't made by checklists, nor can they be made without them. But if your game feels more like the original game than a new one, by a majority of users, then your game is a .5 regardless of the budget, time, and exhaustive spreadsheets and marketing USP's...
I'm not on either side of the argument with GT yet, but I've experienced the .5 stigma as a development leader and it didn't matter how fancy my argument was, it didn't trump others subjective observations enmass.