First of all, Donald Trump rarely puts up much, if any of his own money. Usually he partners with people who are willing to pay him for the use of the Trump name.
Second, he would very likely not be interested in moving into wrestling full time. If he were part owner he probably wouldn't show up much unless tapings or events were in New York.
Third, pretty much everything he's touched has failed at some point. He gets by by leveraging his name, having new partners pony up for him and lots of times, filing for bankruptcy.
Fourth, Trump is interested in getting money back on his investment (if he invests at all). That means he would be making all decisions on what will make him the most money...which would likely forfeit long term growth of younger wrestlers and storylines.
Now, since you said it's hypothetical, let's say that he did but TNA. I'm not sure how much someone like Trump would get over with fans, or bring new ones in. He would for a while, and likely get some mainstream press. But younger fans probably don't care about him and older fans have seen it before. After seeing him for a while it would all blow over and we'd get used to it, media would stop covering it and he'd really just be another commissioner type character.
A guy like Trump is great for a cameo. People know who he is, it's interesting in a "why is he doing this" type of way, and there likely would be major media coverage...but in the long term I don't think it would help, especially if he were an owner.
