I got my 13" i7/8GB/256GB yesterday. I have a 2011 13" ultimate (i7/4GB/256GB) since before, so I ran a few tests.
First of all, Geekbench 3 showed the expected 25-30% CPU performance increase. This will shrink if the machine is really hot, but as long as it can turbo it will be a decent amount quicker than the old machine.
As for graphics, I would have wanted to run a whole host of tests, comparing the two. However, I don't really have time for that.

I wanted to run something, though, and went back to an old benchmark favourite of my, namely Crysis (the original). The following results use the built-in GPU benchmark, running on a Windows 7 64-bit bootcamp installation:
Crysis DX10 Very High (32-bit) 1024x768:
HD3000: 7.82 FPS
HD5000: 14.81 FPS (+89%)
Crysis DX10 High (32-bit) 1024x768:
HD3000: 12.67 FPS
HD5000: 23.08 FPS (+82%)
Crysis DX9 Medium (32-bit) 1024x768:
HD3000: 23.81 FPS
HD5000: 43.41 FPS (+82%)
That's a generational improvement, that's for sure (although still mostly unplayable at high detail levels)! To put this into perspective, I believe this kind of performance is close to an old high-end card like the Radeon X1900XT. It's also around half the performance of an 8800GT. Another way to look at it is that it's current-gen console performance.
Would be nice if more people could test and report their results. If anyone wants to repeat my tests on their machine, it's quite simple:
1. I installed the game via Steam, but the regular DVD install is of course fine as well. Just make sure it's been patched to the latest version.
EDIT: Actually, I think the demo version would be fine to test with as well. I don't think performance changed much in the final version. You can try
this link (direct download).
2. Start Crysis and configure the settings (resolution, detail level) according to what you want to test. Crysis sometimes behaves a little strange when you set the overall detail level. For example if you change from "High" to "Very High" one of the individual settings might remain at "High". In that case, just correct that setting manually.
3. Exit Crysis and go to its installation directory and then the bin32 sub-directory. Run the batch file called "Benchmark_GPU". For a Steam installation, the Crysis directory will be located in [Steam folder]/SteamApps/common/Crysis.
4. The test will run a number of loops and the result of the latest completed run can be seen in the green text to the upper left. You may discount the first loop and report the second or preferably third one instead.
5. Crysis runs in DX10 mode by default. To test in DX9 (like I've done in my medium test), right click "Benchmark_GPU", choose "Edit" and then add "-DX9" (without quotation marks) between "crysis.exe" and "-DEVMODE".
Remember that the results will decrease slightly as the machine heats up, so try to make sure it's well heated before reporting any results. Anything else will be misleading.