That's a scary statement you made about 300 yards of offense. It shows you don't know a ton about football or you are still living in 1978. Offenses have evolved like defenses. There are 129 teams this year in FBS College Football (formerly known as Div 1). If a team this year in College Football averages 300 yards per game, there offense would rank 114th out of 129 teams. Lets say a team averages 350 yards of offense per game (bump it up for the sake of argument from 300 to 350). A team averaging 350 yards of offense per game would rank 100th in the nation in total offense out of 129 total teams. They put up on average 350 yards of offense per game. What do they rank in total offense? 100th out of 129 teams.
I am going to shock you a little more. Lets say a team averages 400 yards of offense per game in football. That team is UAB this year. They rank 75th in America out of 129 teams in total offense and they are considered, "below average" in offense. High School and College are similar regarding offense rankings-yards per game etc. High School offenses that put up 350 yards per game in today's time are average to below average. 1980 was 37 years ago. Now, if a team puts up 350 yds per game and they average 340 rushing yards per game out of the total of 350, that's different. That's usually not the case though.
Therefore, as to whether 300 yards of offense is good enough as your question implied, the answer clearly and overwhelming statistically proven is "it's not anywhere close to being good enough. It's actually poor." On the flip side of that, any team (defense) that holds an opponent at the High School Level or College level under 300 total yards in today's time has actually played, lights out defense. Don't believe it, Miami of Ohio ranks 29th in the nation in total defense this year out of 129 teams. They give up 299.8 yards per game. Their defense is statistically, pretty dang good. If Miami of Ohio's defense gives up 300 yards of game on average in the year of 1990, they would rank 8th in the nation. See my point?