The ABC cut of The Search for Spock that you’re referring to is not extended but actually hideously cut down. Reportedly, the original editing (and this survives in the comic adaptation from DC, by the way) made it so that the Grissom material takes up most of the first act with little cutaways to the other characters until the stuff with the Enterprise in the 2nd act. However, they (rightfully, IMO) decided that it would be stronger to focus on the Enterprise crew for the first third.
Now, the ABC cut does open with the Grissom like the script… But that’s because they apparently cut off everything that happens before.
By the way, there are also records of workprints being shown for both The Wrath of Khan and The Voyage Home. I’ll try and look for the list of extensions for The Wrath of Khan. I saved it a long time ago.
Edit: Here it is.
A few seconds of footage were cut from the simulator scene.
A brief snippet of deleted footage from the scene where Kirk receives his copy of ‘A Tale of Two Cities’ including the statement of Saavik being half-Romulan. Additionally, Spock asks if Kirk is feeling oppressed by something.
A longer take of the McCoy/glasses scene.
Chekov originally had a different log entry, designating he was duty officer. This was redubbed after the workprint.
Carol and David originally had a longer scene with each other, likely more exposition about either Genesis or Kirk.
Chekov spotting a child in the Botany Bay window, and knocking him and Terrell off of a cliff. While the workprint did not reportedly include this take, this scene was controversial in magazines for putting a stuntman in blackface for one take.
Terrell finding Khan’s baby in the Botany Bay.
Marla McGivers is mentioned by name in Khan’s speech.
The Ceti Eel scene was originally a little longer and more graphic. I believe the Director’s Cut has hints of this.
Kyle and Beach (of the Reliant) attempt a rescue, but Terrell (under the eel’s power) convinces them to beam aboard Khan’s crew.
Like the first film, Spock and Saavik originally spoke English in their conversation. This was redubbed after the workprint.
A scene detailing Sulu’s promotion. This is the one George Takei claims Shatner deliberately sabotaged with bad acting- those who have seen it say that was quite obviously the case.
Reportedly, the workprint essentially used the entirety of the Star Trek: The Motion Picture launch scene. This was cut down for the theatrical version.
An alternate take of the scene of the scientists arguing, unsure whether the workprint used the theatrical or director’s cut version.
Carol’s narration during the Genesis tape scene was changed to give a different explanation of the Genesis effect (it was to provide implication that Spock could return).
An alternate take of the discussion in Kirk’s quarters. I believe the workprint was closer to the Director’s Cut.
Kirk had a line about Chekov during the first Reliant encounter.
Scotty is seen telling the other engineers to stay at their posts.
The original cut of the Enterprise’s attack had Khan moving across the bridge shouting at Kirk. This can be seen in an early convention reel.
At one point Kirk recalls the “blind as a Tiberian bat” line while orbiting Regula 1.
McCoy reveals he had worked with Terrell.
The scene of David attacking Kirk was reshot at the studio’s consent. The original featured David leaping out of a cargo container, pulling a knife on Kirk, and actually winning their brawl (this can be seen in the Starlog magazine coverage for the film). This actually reflects a major script change. Reportedly, Kirk had NO idea of his relation to David and learned of it in this scene. The insert shots in the final edit are easy to notice.
Now, this all is more speculatory. The public screening of this workprint cut out during the final battle, right as Spock gets up to go to engineering. The rest of the film included:
A scene showing McCoy working on Chekov’s ear during the final space fight. David is shown to be anxious to get to the bridge.
Chekov later wakes up and demands to be taken to the bridge, David takes the opportunity to follow.
A scene of Khan’s baby in the transporter room next to the detonating Genesis device.
Khan’s final reaction shot before the Reliant’ destruction was cut.
The original funeral scene was slightly longer.
The scene between David and Kirk had different dialogue initially, redubbed at the studio’s consent. “I should be on the bridge” “Are you running away from me?” "I suppose I was."
There was brief scene cut out between Saavik and David on the bridge, paving the way for a later romance.
The original ending lacked all scenes taking place on the Genesis planet, making it seem as if Spock really was dead. Meyer reshot it after studio pressure and a bad test screening. The original edit is referenced on the expanded soundtrack release.