You would see a performance increase, but as Andy said it wouldn't be that much that it'll be worth it.
You can feel the difference in speed in actual use, I can feel a small difference between running my 1600+ at stock speed (1.4Ghz) and at 1600Mhz, at 1600 it reacts a bit faster. But it wouldnt be worth buying a new cpu just to get such a little increase.
Have you oc'ed your 1400? If it's a T-Bird you can both change the multiplier and fsb. No matter what it should be good for at least 1.533Ghz, more if you've got good cooling.