Hi George,
What you found is true, the Goldquest SS is designed to be more sensitive to Gold items than it will be to silver or copper objects. So, what you found is normal. This has to do with the pulse width. The GQ SS has a shorter pulse width which works great for gold, but not so great for more conductive objects such as silver or copper.
Now, as for your question about what a person should expect for depth of finding 1" copper/silver objects, my guess is you should see about the same depth as you could get on a US nickel, which is made of a lower conductive material. The US quarter is about 1" so a quick check can easily be made. The depth could range from about 12" to maybe 15" and would depend upon external noise, ground conditions, coil size, etc.
In most cases, the 12" to 14" range would be more realistic, with the 15" range in areas where everything is extremely quiet, noise wise and the operator has the threshold set right. Don't be surprised that on noisy days, the 12" depth range is difficult to obtain. In many cases, the noise is not obvious but can have an obvious effect.
Reg