Confirmed: same Snow Leopard slowdown on a brand new iMac that shipped with Lion. As zirkenz points out, it is significant that the issue is not confined to the Mac Mini.
Details:
iMac 27" 2.7 GHz (iMac 12,2 quad core i5)
Geekbench scores (32-bit, freeware version):
Lion (10.7.1): 8000
Snow Leopard (10.6.8): 4400
For comparision, the same hard drive volumes were used to boot and benchmark a MacBook Pro. There was no difference in results:
MacBook Pro 2.4 GHz (MacBookPro6,2 dual core i5)
Geekbench scores (32-bit, freeware version):
Lion (10.7.1): 4980
Snow Leopard (10.6.8): 5000
In addition, two other tests also showed a reduction to about half-speed on the new "downgraded" iMac: (a) a short, custom C program that runs a Monte Carlo and counts MFlops and (b) Xbench. For each, the overall and detailed times are about double in Snow Leopard compared to Lion. (The Lion benchmarks are at the level expected given the hardware and other published results, so it is a Snow Leopard slowdown rather than a Lion speedup.)
The Snow Leopard installation was cloned from a MacBook Pro, updated with combo to 10.6.8. (Also repaired permissions, zapped PRAM, and checked with additional physical drives). iMac was bought for use w/ Snow Leopard, so Lion was an unwelcome surpise. Apple Store had said the machines were still shipping with SL; now, however, Apple is unable or unwilling to provide the gray install discs. So there is no way for now to do a clean install of Snow Leopard, which might have revealed if these repeatable results are due to a software issue or due to a change in the hardware since Lion was released.