You've ALWAYS got to take boost into account. That and intake valve area. Bigger intake valves require more spring pressure, since the boost acting on the valve has more surface area to act upon, and therefore "takes away" a larger amount of seat pressure. Also, stainless valves will need more spring pressure than a titanium valve will because of the mass of the valve itself. Of course, rpm range comes into play, too. More rpm requires more spring pressure.
For a stainless valve with a diameter of around 2.20 inches in a turbocharged application seeing 20 psi boost pressure @ 7500 rpm, I would want absolutely no less than 325 lbs on the seat. Titanium valves buy you the ability to run less pressure (at that boost level), but don't mix well with extended street driving. More boost will require more spring.
As Tom alluded to, there is no free lunch. In one area or another, we must all pay the piper. There is always compromise when putting together a high powered engine, especially one that's going to be run on the street. The question is can we tolerate negative aspects of that compromise. Turbocharged engines allow us to have enormous power levels in street driven applications with less negative compromises.
Still, there is no free lunch.
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