True turbo "lag" is purely a throttle response issue. To see lag, sit at steady state, at some rpm above the boost threshold, and nail the throttle, the time it takes to reach full boost is lag. Lag really isn't determinable by a typical dyno curve.
Boost threshold, is an rpm level, at which the engine is finally making enough exhaust energy to spool the turbo properly. ie, stomp the throttle at 1500 rpm in just about any car, and it won't make boost until it gets above the boost threshold.
It takes exhaust energy to spin a turbo, some heat, some pressure, some velocity, if you don't have enough exhaust energy, the turbo just won't spin fast enough to make "boost" . On a similar note, a wastegate, simply bleeds off the extra exhaust energy (routes it around the turbine) once the desired intake pressure level is reached.
That said, if you want high horsepower at the top end, you want a turbine and housing that can use every bit of exhaust energy at that higher rpm. A turbine housing that can make use of all that exhaust energy, is typically too large at the lower rpms, so you end up w/ a higher boost threshold than you otherwise would.
That is the case here, Turbo Tom desired a turbo that would make better use of the exhaust energy at the higher rpms, and therefore build more power, so he must take the tradeoff of the higher boost threshold. Or actually, if I read an earlier post correctly, he desired the higher boost threshold in order to make the best of his limited traction situation. Whatever floats your boat; its all good as long as things work out the way you designed them too.
