Yeah John had appearances in Johns' early run, but he didn't really use him as a prominent character (or even a real supporting character). In his early run, John was put on an irrelevant mission that had nothing to do with the main story. There was no real purpose. His appearances were mostly just cameos and miscellaneous roles. The same applies to Sinestro Corps.
From the top of my head, John was in the Agent Orange story arc, where he didn't do anything note worthy there, other than getting his ass kicked.
He also appeared in the Zamaron story arc, where Johns tried to pair him and Fatality together, where that little build up didn't go nowhere until the end of Tomasi's run years later when he took over John. And then Van Jensen added further developments to it and fixed it (because it made no sense). It was really a half-assed development in Johns' run.
He appeared in the Alpha Lanterns storyline where he decline an offer to be an Alpha Lantern.
He also appeared in a cameo appearance fighting Hal in Hal's origin.
He retcon John to be a stoic Marine in the Blackest Night story line.
These are just from the top of my head. Basically removing John from all of that wouldn't have made a difference to the story direction and the Green Lantern mythology.
Green Lantern Rebirth and the single issue of Blackest Night is the most Geoff Johns ever did for John and that's not really saying much. Everything else is just crumbs and irrelevant stuff.
As for Peter Tomasi, Guy Gardner is his favorite Green Lantern character, which was why Guy was the most prominent. It's also the reason why in Green Lantern issue zero, Guy had an origin where John was not present in that issue and had no origin. The time John had a big role in Tomasi's run was when he was captured as a POW and Guy Gardner had to rescue him. The story lead up to the next one where John was then put on trial for killing another GL. So that's the biggest role John had in Tomasi's run. Guy was the real star there and the most heroic.
Now the most John has ever done as far as making a real difference is the recent stories.
This all started when Van Jensen and Robert Venditti took over the franchise.
John helped rebuild OA on Mogo.
Trained new recruits.
He saved the GLC by restoring the Power Battery in Lights Out!
Became the first male violet lantern, because of his strong love for the Green Lantern Corps. Which then lead to a successful rescue mission and saved the Zamerons from being killed.
Saved a planet from being literally destroyed in the end of Van Jensen's run.
Quickly ended the conflict between the GLC and Sinestro Corps by adding a failsafe Sinestro Corps rings.
It was fixed that John still had his Guardian powers.
He became a GOD and defeated Koyos, restored a destroyed OA back as new again, which saved the entire Corps from being destroyed.
So yeah, he definitely made a real difference to the mythology as a character after Tomasi's run.
This was all because of his leadership and skills as it transitioned to later stories like we seen in Cullen Bunn's run, Robert Venditti's Hal Jordan and the Green Lantern Corps, Robert Venditti's Justice League, to now in Redjack's run
If anyone says John didn't make a difference to the mythology post Johns' era, they either haven't read recent stories or is lying lol.