As it turns out the PS3 Slim has a Toshiba MK1255GSX (HDD2H26) 120GB, 5400RPM, 8MB Cache, 2.5 inch, 9.5mm Hard Drive. Not the exact same as the PS3 fats does (Seagate 5400 8MB Cache) but similar enough that it leads me to believe the firmware and other efficiencies are at the root of this increase in performance.
So it looks like if you put a 7200 rpm hard drive into a PS3 Slim you might see even better performance.
Here is a link to an article showing performance increases with putting a 7200 rpm drive into a PS3 fat. http://www.gamepro.com/article/features/138288/7200rpm-vs-5400rpm-does-ps3-hard-drive-speed-matter/











