You are right on Joe.Take for instance the .41 A/R housing (stock volvo). Changing turbine A/R has many effects. By going to a larger turbine A/R, the turbo comes up on boost at a higher engine speed, the flow capacity of the turbine is increased and less flow is wastegated, there is less engine backpressure, and engine volumetric efficiency is increased resulting in more overall power. The .41 A/R is able to create the pressure differential at a much lower engine rpm, giving the compressor ability to make its maximum rpm speed sooner, why the 15G and all others spool very quickly. As the engine rpm climbs, the pressure differential is lowered due to the physical volume of the housing size becoming a restriction on the post turbine side - volvo turbine housing suck !. As the housing size is increased, it take greater engine rpm speed (greater exhaust energy) to spool up the turbine, but the pressure differential is less effected by the physical volume of the housing. If you are after maximum midrange gains smaller housings are essentially (15G,16T,18T), if top end gains are essential larger housing are essential(16G,20G,3071R, 28R).