I emailed Mike of the "What Goes On" website - http://www.pootle.demon.co.uk/wgo.htm
- and he has now entered the debate too! Here is his reply:
'Another "Who sang SLYYYY" brewing?
The opening section of "Ah's" to 2:53, has qualities of Paul's voice, but
not beyond that point. The ragged ending at 3:12 is more John's style.
From a recording point of view, the quality of the voice and effects matches
John's from the rest of the recording (huge delay and echo, being panned
around, thin and harsh) -- contrast against Paul's fairly dry, close mic'd,
bass present voice.
Also, Lewisohn's _Recording Sessions_ states that Paul's bit began at "Woke
up" and ended on "everybody spoke and I went into a dream .... oh sh*t" --
which was later replaced with a take *without* the vocal error and expletive.
I'd go with it being John ... there is a background vocal also around 3:03
to 3:10 singing "oo-ooo" over John's vocal, I think that might be Paul. Just
to add confusion.