Also, Len Wein and Dave Cockrum openly admitted that Sunfire and Thunderbird I were to be "flunked" off the X-Men. None of the staff liked Sunfire so he was immediately ditched.
COCKRUM: "Kind of at the last minute. The way this all came about was that when we were first planning out that first issue, we decided what we were going to do was have it be an aptitude test or an entrance exam or something like that. They would be sent off to rescue the original X-Men, but the original X-Men would not actually be in any danger. We figured if it's an entrance exam, theoretically, there are people who are going to flunk as well as people who pass, and so we had Banshee and Sunfire, and we were going to flunk 'em. Then we thought, well, that doesn't seem fair, we ought to have a new guy to flunk too, a new guy who's unsuitable. So that was what Thunderbird was for, to be a flunker. He was unsuitable because he was anti-social. Hah! As if Wolverine's not anti-social. But at the last minute- well, I liked Banshee and we all liked Thunderbird, so we figured to hell with it. It turned out not to be a test anyway. So we had Sunfire, who nobody much liked, go off in a huff, and we kept Banshee and we kept Thunderbird. But then we didn't know what to do with Thunderbird because we never thought him out. It was easier to kill him off than to think him out."
The only divergence that Sunfire would have caused had he remained as an X-Man would have been during the Phoenix Saga. Instead of Jean, Shiro would operate the shuttle since his powers would protect him from the radiation in the reentry while Jean could psychically implant Doctor Corbeau's piloting skills into Sunfire. So Jean may not have become the Phoenix which would lead to serious repercussions in the upcoming conflict with D'Ken.