It looks to me like the solution to the problem is simple. Just have a "high tech visa" with no number limit, but one simple requirement: The company requesting the job needs to be offering something like 150% (or 200% or whatever makes sense) the average salary of the US workforce, or in their state.
This will ensure that the visa will only be offered for skills that are difficult to find in the USA (or the specific State). No problem of driving salaries down.