AT the moment CT Profiles seems to be the only way I know of of trying on new gear and seeing how my stats alter. I would really like to see a mod for this if there isn't one already. (And if there is please tell me!)
Rating Buster shows the stat difference for individual items on the item tooltip compared to what you are wearing in that slot.
In terms of an in game mod which basically does the job of ctprofiles I don't think one exists yet largely because I think it would be huge undertaking.
Keeping a db of item id's up to date wouldn't be fun and worse keeping formula's for every stat for every class/talent/race combo would be even less fun. As you can see with ctprofiles keeping everything up to date and getting it right for every class with the complexities of sockets, enchants and set bonuses is a bit of nightmare and in the end if the stats are wrong/unreliable the value of the addon is nothing imo.
Don't get me wrong I like the idea of an in game mod that's a character sheet clone/replacement where you emulate wearing gear and calculate the stats for that set (in game ctprofiles). I just think the potential for it worthless because it's wrong is quite high.
Now... we just need someone to prove me wrong and develop it... but who has the time thesedays!
Anyways, when I read the OP, I didn't think at all that it would need to track all the particulars like attack power and such other than what the items themselves gave. I mean, I interpreted it as changes in strength, agility, intel, and such, and if items gave direct bonuses to AP or defense, then it would show up. A mod following this interpretation would not be as hard as you described, right?