A 300 gun will do the exact same damage to a 30 and a 28. So yes, a 300 damage gun in the new nightfall will do the EXACT SAME damage as it would have in 28 nightfall.
The only reason to have a 300 gun previously was the hard mode raid. There was nothing in the game above level 28 outside of that. So if you didn't do the hard mode, there was no reason to level a gun to 300 outside of iron banner.
I'm certain this isn't true. I kept my IB at 280 or whatever is one away from 300 for ages and my 300 Supremacy always did more damage in Nightfalls, when I upgraded my IB, it surpassed it.
To be honest the damage of these weapons won't be a problem. Destiny is easy as shit and bosses are damage sponges regardless. Being more powerful doesn't affect difficulty since strategy is based on patience and not being fucking bored shooting the 300th Sniper Round. But the reason people kept playing the same shit was because they felt like they could make progress. Bungie's update takes the progress we made and give a big fuck you.
Xur will upgrade random, or well, Bungie chosen shit, whether we worked on it or not. That means that The Last Word sitting in someone's back that they never used will be more powerful than the weapon they loved because Xur is selling that uograde rather than the one for the levelled up weapon.
I got back in because the patch was benefitting players. Now I understand why it is there. This is fucking stupid. I really wish I never played it. I have wasted my time on so many games in my life but Destiny makes me angry and Bungie know full damn well the kind of skinner's box they have created.
Destiny is no better than nicotine. Just put some nicotine on Destiny's game box and lick it once every so hour. Soon you will be coming back to it day after day, licking the box but not being made happy from it.