Depends on the CPU load....
If your i7 is idle (say <10% CPU load) the difference should be more like 5 C. At the moment my MBP just has Safari open and the difference is 5 C.
If your i7 is working hard (near 100% total load or 4x100% for the 4 virtual cores) the difference will be very much like 20 C.
My 13" 2009 model Core2Duo is 20 C hotter than the heatsink when at 100% CPU. My 2006 CoreDuo Macbook is similar. I've seen others in person with the same difference, also some posters on here have reported almost exactly the same answer.
If you do the math for the thermal resistance of the silicon substrate, the copper spreader and the thermal paste, you'll come up with a similar number.
Was your MBP idle?