Yes, that's a "Win/Win," although the "real world" says a 20% return on investment is great in most instances.
This is a little different that other investing scenarios where you are investing by yourself, because in this case you are investing with a partner. The backer and the player are partners together in an investment. So you not only have to take into consideration the potential return on your investment, just like always when you invest by yourself, but you also have to consider how the return you are getting is comparing to the return your partner is getting.
It isn't fair if one partner is raking in money from an investment while the other partner is losing money, if what they both contributed to the investment was close to the same. It wouldn't even be fair if they were both making money, but one was making a whole lot more than the other. My personal opinion is that the backer and the player are both contributing about the same. The backer provides the money, and the player provides the pool playing. I know this can be argued, like it could easily be argued the backer's contribution is the more significant one since he is the only one that can lose anything in the deal, but lets still call it the same for now.
A reasonable argument can be made that over the long haul of their arrangement, one of them should not be making a whole lot more money than the other one. If one is making money, and one is losing money, I think everyone agree that it isn't a very fair partnership at all. Even if they both end up on the positive side, if one only makes a little bit, and the other makes a whole lot, I think most still agree that it isn't a very fair partnership. The bottom line is that the amount of money that you actually get to put into your pocket is the most important thing in determining how fair the arrangement is for both partners.
With a 50/50 split, the money that each one actually pockets in ludicrously unbalanced, and generally the backer is not even pocketing money, but losing it. Here is just one example of an arrangement where both the player and the backer make about the same amount of money, which would be fair by many people's standards. If the player averages being about a 62.5% favorite in the matches you back him in (which means he wins almost 2 matches for each one he loses), then at an 80/20 split, with the 80 going to the backer, the player and the backer
will each make the exact same amount of money over time (presuming a random assortment of amounts being bet of course). So if you back a player that averages a 62.5% win rate (almost 2 wins for each loss), if you feel it is fair for you to each make about the same amount of money, the correct split would be 80/20. If the player wins less often than that, then the correct split gets worse for the player. If the player you back wins more often than that (which isn't very common), then the split gets a little better for the player.