A friend of mine has a 1.25 15" and I have the new 1.33 12". We ran XBench on both of them, and found that the scores were VERY similar. I recall my 12" beat him by about 2 points, which is not much. He had 1 gig of memory, and I have 768k.
FYI: Photoshop CS runs like a dream on this machine!