Pretty sure the 6830 didn't have a 23 W TDP. In another thread a few people were claiming the TDP of the 6830M was only 24 W, but someone else said that AMD only says the TDP of the graphics chip alone and does not include the other things in the TDP, whereas nvidia gives the TDP of the entire thing. I find it hard to believe a GPU as powerful as the 6830M only has a TDP of 24W. The 5830M has a 24 W TDP, but the core clock in it is slower.
The 6830M also uses DDR3 for the memory and Apple usually uses the superior GDDR memory, in this case GDDR5.