What about for efficiency? EMP is supposed to be reduced closer to 1:1 or lower and you're converting more of the exhaust's energy to work (like a compound steam engine).
Altitude as well. I live at 5200' elevation and many highway passes are over 10,000'. High altitudes drive up the pressure ratio and reduce the airflow. A single turbo, especially a VNT as in my case, is right on the surge line most of the time and its maximum boost is limited by that surge line. My turbo is capable of 28psi as a single, but that dang surge line forces me to limit boost to 15psi.
I need a low pressure compound (3-5psi) to reduce the pressure ratio which will allow me to keep well clear of that line and reach my desired 20-25psi boost range.
I need help figuring out which LP turbo would be best for my needs.
The turbine needs pressure across it to work, at 1:1 pressure ratio (turbine inlet/turbine exhaust) there's no 'blow' to spin the turbine! The prob with compounding, as your thinking, is automotive sized turbocharger turbines need a pressure ratio of 2:1 to do any work at all, and 2.5 or 3:1 to peak efficiency. Drive pressure is compounded (multiplied) just as boost pressure is; so you'd have higher exhaust pressure (EMP) with compounded turbos, to get the same boost (28 psi), than you'd have with the proper sized single turbo (about any single turbo can do 28 psi easily, in the appropriate situation/installation). Small sized automotive turbocharger compressors will do 2.5 to 2.8:1 pressure ratio at max efficiency, and large automotive turbo compressors will work at 4:1 with good efficiency (aircraft jet engine sized centrifugal compressors can do more than 8:1). Compounding is necessary, and efficient, when you want/need an intake(boost) pressure ratio that is higher than can be achieved with a single compressor stage. (To do a set-up that compounds two low compression ratio compressor stages to make a single-stage-level pressure ratio (28 psi), but has two turbines driven at regular, 2:1, turbine pressure ratios (about the min that will work), is prob off-design conditions for most typical automotive application/size turbos)
Turbochargers are (for all practical purposes) altitude independent; turbocharged piston/recip aircraft engines demonstrate that, to very high altitudes. Turbine efficiency increases with increased altitude, and compressor characteristics (maps) stay the same, except that compressor speed increases as intake density decreases (altitude increases).
If you have a compressor surge problem, certainly that's a wrong sized turbocharger. Turbos are designed with well matched turbines and compressors; more or less, the mass flow into an engine is the same as the mass flow out of it. VGT, VNT etc doesn't change the compressor, or turbine; the variable turbine inlet geometry is only a means of keeping the turbine pressure ratio high (or higher than it would other wise be) at low mass flows (low engine rpm).
Sequencial turbo charging is what you need! About any automotive sized turbo can make 28psi efficiently; you just need a very small primary, and a way to by-pass it so the secondary turbo won't choke at high flows.
There's efficiency ... and there's efficiency. Reducing compressor PRs across two compressor stages could increase compressor efficency, compounding the exhaust back pressure with two turbines prob doesn't increase the engine's efficiency, viscious losses with the extra flow paths prob doesn't help, either... FWIW, any charge air cooling just throws away some part of the energy that was recovered from the exhaust by the turobcharger...
