Since you're asking for an expert, I'll have to fake it.
Performance increase is dependent on what you're running that you more you need. The i3 is a dual core CPU, but it does Hyper-Threading. That means the CPU will tell the programs that there's 4 cores so that it gets 4 streams of data to process (or threads.) Since the data to be processed usually isn't constant and there's time the CPU isn't doing anything while its waiting for information to process. So if the applications are providing 4 threads, each core can bounce back and forth between the pairs of threads during that down time and spend less time idle. If you're doing HEAVY multitasking, being able to run 4 threads is better than 2. But if its just general surfing and productivity applications, probably wouldn't notice a big increase in speed over either of the other 2 CPUs. The core i3 mobiles are also 35w CPUs and have the graphics on the CPU. So power consumption would be more than the P8400 but less than the T6600.
If you already have the T6600 or P8400, you can't upgrade to the i3 mobile as it uses a different socket and motherboard chipset.