That's probably pretty close for the sea level number, and I don't think it will drop off by a factor of 2 at your altitude; maybe only drop off 20% or so. It really depends on how well your springs and lifters work together. On my 428CJ dyno mule, when I swapped the hydraulic lifters for solid lifters on the Edelbrock Performer RPM cam, the horsepower continued climbing for a few hundred RPM past 5300. With the hydraulic lifters it had always hit 5300 and then fallen off. At a speed of 5800 RPM there was about a 30 HP difference between the solid and hydraulic lifters. There's a graph of the two horsepower curves in my book.
If your valvetrain setup hangs in there to 6000 RPM with the hydraulic lifters, then you probably won't gain anything with solids; you might even lose a little. But if its a run of the mill hydraulic cam setup, I doubt it will go past 5500 before it starts to have issues. If you don't want to spend money on a new cam and lifters, I think you might consider a spring upgrade to keep the lifters under control up to 6000 RPM (assuming you haven't done that already).
