while he's arguably a hybrid PF/C, Duncan is officially considered his team's PF.
You specifically mentioned Splitter as a post-up threat against Miami, and I think that's a big stretch. He's around Bosh's height but the key thing is he's not going to push Bosh around with ease as happened in the last series.
Although I wouldn't bet the house on Bosh's toughness inside.
I don't think that will come to play since Bosh does not score in the paint as often. Splitter and Bosh wont be "banging" each other too much. The Spurs and Heat both like running the pick and roll with Splitter and Bosh, respectively. The difference is that Bosh likes to fade away after setting the pick to spot up for that 15 to 18 ft jumpshot. If he gets the pass - he'll most likely shoot that jumper. If he doesn't - then his role ends right then and there because usually someone's scoring or he's just too far away to be crashing the boards.
I don't see Bosh rolling towards the basket as much, especially when the paint is clogged. Who knows - maybe since Hibbert is no longer there, Bosh/Heat will feel confident enough to post up down low. i doubt it. The Heat don't like to run your traditional post offense sets.
Splitter doesn't have the range that Bosh does but Splitter also doesn't look to score - Spurs will go to Splitter only if there's an obvious mismatch. Even then he looks to pass..Splitter will still be a threat in the post - but as a passer more than as a scorer. If he's open off of a pick and roll, Splitter will look to make that extra pass to either TD or the 3pt shooter in the corner (Danny Greene & Gary Neal (he'll play more)).
Obviously, as individual players Bosh is better than Splitter but Splitter will play a more crucial role in the offense (as long as he doesn't get in foul trouble). Only way he won't is if the Heat score bunches in transition as Bosh will surely outrun Splitter. The Heat SHOULD post Bosh up as much as they can vs the Spurs - but I doubt it. Spo will stick to his small ball.