Current lineup:
13": 17W CPU + ~12W 320M = 29W
11": 10W CPU + ~12W 320M = 22W
Sandy Bridge lineup:
13": 25W CPU + 4W PCH = 29W
11": 17W CPU + 4W PCH = 21W
The TDP of 320M is unknown but 9400M has 12W so I'm using that as the TDP. Anyway, give or take a watt or two, it's not going to change anything. 320M includes the functions of chipset controller thus there is no need for another chip to handle the I/O and other stuff. Sandy Bridge still needs one though (PCH).
When just looking at the TDPs, there is no reason why SB MBA would generate more heat and noise. Obviously, the SB MBA has to come out before any conclusions can be drawn.