Here are some numbers FWIW.
My Impala has 177k miles on it and dyno'd (at the wheels) 286 rwhp 335 rwtq.
It only has tri-y headers, cold air intake, underdrive pullies, and a tune for 92 octane. The valvetrain is all stock. That is it.
Some friends have a similar setup with the GM Hot Cam and getting in the 320's. Another friend has a 383 CID with ported stock aluminum castings and a mild cam which is putting down 398 RWHP and is very driveable.
The Impala's came with the iron head LT1 and a little less aggressive cam, so they were rated at 260hp (crank) by the factory.
Like a lot of engines, some headwork and a decent cam will really wake these things up. It is ju$t a matter of how fa$t your ca$h i$.
Randy