if you are looking at ntune you are probably in 2d mode, and therefore it might be running slower? trying running ntune leave it going in the background and then starting a game for a little while, stop the game and see if the graph stepped up.
Rivatuner does still exist, and it will also log whilst you are running games so that you can see what is actually happening.
The other most likely cause is a lack of power causing the card to underclock itself.
No advice, other than the speed of each part of the card is linked, i.e. if you raise one part by 13 then something else will go up by 5 or something like that. Hence its kind of steppy, where theres no benefit for a rise of 12 but there is for 13, the actual speeds of each element are only inferred I think and you can only change one element, everything else just follows. I've not bothered.
I'm surprised that ntune doesn't give the right answer.
Get Riva Tuner 2.09.
I found where the clocks are listed and you can adjust there.
Click the corner icon in the drivers dropdown box and select the first graphic setting icon that looks like a card, this is the driver settings (bottom half of the screen you will start on) to get there.
My 7950GT 512 is 565 Core, 715 Memory, I am not OC'n it, performance is too good as is, IMHO.
I'm very happy with it's performance in Age of Conan.