Does this also help with turbo lag?
No idea. But in the case of a semi truck, where engines don’t spin up much more than 2000RPMs, I think turbo lag is a moot point. This is more about fuel efficiency than anything else.
The fascinating thing about it is that a compound-turbo engine is the halfway step before going to a turbine engine. The boys at detroit diesel figured out that adding a compound turbo after the regular turbo increases the effectiveness of the new pollution control equipment required by the new stricter pollution regs.
I can’t wait to see what they come up with in the next 20 years.