Air has an ever changing spacific gravity. Barometric pressure, humidity, temprature. All have an effect on the density of air. New-style compressor wheels have inducer blades to assist air into the more aggressive blades further in. The shorter blades are called "splitters". Oh, and were talking shaft speeds in excess of 240K here not 60K to 100K. Depending on blade-to-housing clearances, boost can vary. Supersonic? absalutely!!!! If throttle blade(s)close suddenly this can dead-head the turbo, leading to sheering, as the blades cannot continue to pack non-moving air. This is where a bypass valve comes in when designing a turbocharged system. They work by vacuum, ported so when the throttle blade is closed, vacuum is instantly present.
What I beleive you are asking above, about the shock wave is; surge. At wot a too-small turbo compressor wheel will exceed it's abuility to grab air. Even a wheel with inducer blades has a "range" of operation to follow. We call this a "curve". Think this through at super-sonic speed. The first blade swipes air leaving nothing for the next blade to grab, nor the blade behind it. In an instant, the wheel is free without resistance from drawing air into itself. Shaft speed shoots up out-of-control. In time, boost drops off because exhaust pressure no longer has as much force driving the turbine. As shaft speed drops, the compressor blades grab air and control is regained. Boost shoots back up, the turbine speed increases and once again the compressor wheels blades loose traction on incoming air and shaft speed spikes back up. Over & over until either the shaft breaks or bearings weld themselves to the shaft, which also can break the shaft. If the shaft can withstand this sonic pounding, some do, the nut can work loose on the compressor wheel and fall off. Plumbing can shake loose on the system as well. This is called "surge". It can happen so fast all you hear & see is smoke and parts going every which way.
Compressing air generates heat, it's a fact of life we must accept. Controlling this heat can be as simple as installing a heat exchanger or intercooler. How efficient the intercooler is depends on how well it drops tempratures with minimal air restriction. Which is never as good as no intercooler.
<small>[ March 17, 2004, 10:50 PM: Message edited by: Ryan ]</small>