PI-efficient engine upgrades and swaps

I raised this topic on some of the Forza communities on reddit, but I thought I’d raise it here too.

As far as engine swaps and upgrades go, I know that some are more or less PI-efficient than others. For example, upgraded cams are thought to be less PI-efficient, whereas the centrifugal supercharger is (often) thought to be very PI efficient. I’ve also found that in B-class the 2.0L F4 Turbo Rally is extremely PI efficient, so long as you’re using manual with clutch, lengthened gears, and know to shift at 5.2k rpm (which is well before redline). You’re effectively getting +20-30HP for free, which goes a LONG way in B-class.

But beyond these examples, I don’t have a good sense of particularly PI-efficient engine upgrades. So my questions are:

  1. Are there other particularly PI-efficient engine upgrades?

  2. Are there other particularly PI-efficient engine swaps?

Some engine swaps, might be PI-efficient just because they allow for adding a centrifugal supercharger, which is PI-efficient. So, I am also wondering:

  1. Are there other engine swaps that are particularly PI-efficient, but not because (or not just because) they allow for adding a centrifugal supercharger? (The 2.0L F4 Turbo rally I mentioned above would be an example of this.)