Can't say for a B777, but I suspect the principle is same as B737. APU to packs = APU supplying air for pressurisation and air-conditioning. Some a/c might allow this to be via 1 pack or multiple. Packs Off takeoff could imply unpressurised. Remember, the APU can be u/s. The performance improvement depends on engine bleeds on/off not being pressurised or not.
I think VNAV PATH is confused.