The gasses will exit faster the more there is (if your running higher boost for example) as the pressure in the chambers will be higher, also it may not be the gas velocity that effects the spool up time in this case, we have increased the size of the turbine housing and so maybe it needs a greater volume of gasses to spool it up faster (which is fine as we are running higher boost) but perhaps the ports are not big enough to flow this extra volume fast enough............ this is what I'm thinking any way!