Genuine question (If I may!) What was the motivation for switching from N3 to N2?
I never worked the 787 and the Trent 1000 (although I did follow the GEnx-1B engine program fairly close since it was so similar to the GEnx-2B that I was responsible for on the 747-8). I'll always remember when we held CDR for my system on the 747/GEnx-2B, I got up and said that our plan had been that the GEnx-1B would be flying around, generating experience and data, before we got to this point on the -2B (the 787 program had slid so far that 787 first flight was still nearly a year away at the time
). My counterpart from the GEnx-1B looked at me and simply said "Bad Plan"
Anyway, I seem to recall they were concerned about pulling all that gearbox power (and those two big starter/generators) off the high pressure spool and thought it would be more efficient to pull all that power out of the intermediate spool.
Something the PD people seem to forget is in most cases the devil you know is easier to deal with than the devil you don't know...