You might see more of a difference using an SSD so the CPU isn't waiting on the storage system as much.
I haven't seen the performance comparisons you're referring to, but my understanding is that TrueCrypt makes use of the AES-NI
where available, and that likely makes a huge difference for Sandy Bridge... provided you're using a SB variant that has the AES-NI feature-box ticked. Alas, thanks to Intel Segmentation Hell, many do not -- particularly at the low end (AFAICT
the I5-2500M line has it, the i5-2400M line does not, nor does the I3 line so far) -- so depending on which SB system you happen to be looking at (and whether the comparison you saw was exploiting AES-NI), Sandy Bridge may not have as much advantage as you think.