Pipes could easily account for 2 Hp.
While the lengths are different, what is important is the actual length of each pipe from valve seat to to end of the pipe and weather this is a pulse node to not
Thinking about it, I think that high level system was probably designed at Umberslade.
The boffins there were quite good at the theoretical stuff like pulse nodes, so maybe it does give a boost.
I read a 1969 (I think) road test which slated the Firebird and it's fangled exhaust system for ruining the performance of the twin carb engine.
But that was just conjecture, another fault could have been causing the problem instead.
Yet another road test got 115mph from a Firebird
That one blew its gearbox apart at maximum speed!