Based on the demo's performance on my 2010 MBP with the 330m GPU, you might be able to play it on Medium with the 6750m GPU. The improved processor will probably also help prevent any stuttering relating to large armies.
I had to play it on Low settings to have an acceptable frame rate. It looked like crap when zoomed out of the battlefield to see the big picture, but was pretty cool looking zoomed all the way in. The improvements with the individual soldier models doesn't help the look of Low settings, because the added shadows contribute to the men looking worse than 8-bit blurs on the screen when zoomed all the way out. It was a big disappointment to see it, but I don't disagree with their direction. More lifelike battles are going to need beefier GPUs to show them off.
I'll be passing on Shogun with the 330m. The Low settings are not worth using just to be able to play the game. Poor graphics kill the experience on Total War games, especially when you know you've turned off so many bells and whistles just to play it.