Comparing the ZVH is not really applicable as an extra 200CC can make a huge difference, ZVH are well known for fast spooling times for this very reason.
Rob if you bought a .48 housing it would make it a stage 2 unless your compressor wheel is bigger too? Although I would imagine it would behave more like a stage 3 as it's the turbine side that plays the biggest part in lag.
I agree about the volume of gasses in the chambers being finite but my thinking was that the quicker the engine can exhaust them the faster the turbo will spool, and in turn the faster it can take fresh gasses in, making the whole engine more responsive............ this is of course to the point where gas velocity drops too low and you get the opposite effect, and of course if the ports are restrictive in the first place!?
If it is this whats causing the lag, in conjunction with the agressive cam profile (or maybe lack of profile as it's done a few miles?!), then maybe some porting is the way forward!?

FYI it makes 7psi by 3000rpm, 15psi by 3500, then peeks at 24psi bang on 4000rpm.