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Yea if you look at charts that graph in cylinder pressure as a function of crank angle you'll see that when the exhaust valve opens an NA engine is at about 3 bar. It's not that much pressure if you think about it, adiabatically expanding that only gets you a pretty small amount of energy.
Turbochargers all increase backpressure. The problem is that the high pressure exhaust gas blows out of the cylinder in a short wave rather than continuously, so when it hits the turbine housing it gets reflected back and stuff. First of all this limits how much energy can be extracted, which is why turbocharger housings add restriction to increase the shaft power available for the compressor to use.
The other issue is that a turbine is not actually converting heat into work, it is only scraping up leftover uncollected work, if you get what I mean. Expansion in a turbine is adiabatic, the amount of heat energy ideally stays the same while the net "work" done on the gas goes down (pressure drop). In real life some of the work turns into heat because of friction (blanket term for all of the losses).
If we run some calculations or look at F1 turbocompounding (which nets 7% efficiency gain), we see that at full load the engine is spitting in the neighborhood of 10% of its output into the exhaust pipe in the form of useful kinetic energy (pressure). Since the gas engine (non-Atkinson cycle) is running at ~35% efficiency optimistically, the pressure leaving the exhaust only accounts for 3% of total fuel energy. Using the 30% of fuel energy going out the exhaust figure, we see that the vast majority of the energy is leaving in the form of pure heat.
Something like the turbosteamer will cause a little bit of restriction because it cools the exhaust and perhaps reduces the exhaust gas velocity a bit. However the exhaust valve opening before the bottom of the stroke means that most of the exhaust leaves before the piston even hit the bottom, of its own accord. Scavenging does not deliver very much energy back to the piston since the wave coming back is weaker and needs to go through the exhaust valve anyways. I believe the gains in a custom exhaust system are from 1. removing excessive muffler restriction (sometimes the cats :O) and 2. tuning for pulse interference and scavenging with the headers, but the stuff that happens before the collector that merges the pipes together is what's most important since that's where the pulse effects are strongest.
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