Simple player rating system
Create a simple rating system which expresses player performance as a function of actual winning percentage vs. expected winning percentage.
Simple example: If you were expected to win 25% of your games (based on the number of players/teams in the games), and you actually won 35% of those games, your performance rating would be 35% / 25% = 1.4
I think this method has some advantages over something like an Elo rating system. Particularly the fact that this rating is very easy to calculate (elo for a multiplayer game like Warlight is even more complex than for a two player game). The average performance rating would, by definition, always be one (1.0). This is nice because Elo systems tend to have rating inflation issues over time. The one downside is that opponent strength is not taken into account, but this will tend to even out somewhat over time. And even with this minor drawback, it's still a huge improvement over the raw winning percentage that is currently shown.
Full example  numbers in brackets are [actual winning percentage / expected winning percentage]:
2 player FFA: 2 / 5 [40% / 50%]
3 player FFA: 3 / 8 [37.5% / 33.33%]
8 player FFA: 3 / 20 [15% / 12.5%]
2v2: 5 / 8 [62.5% / 50%]
2v2v2: 3 / 12 [25% / 33.33%]
3v3v3v3: 2 / 5 [40% / 25%]
Then you calculate the overall performance rating as a weighted average using these numbers:
First step:
((40% / 50%) x 5) + ((37.5% / 33.33%) x 8) + ((15% / 12.5%) x 20) + ((62.5% / 50%) x 8) + ((25% / 33.33%) x 12) + ((40% / 25%) x 5) = 64
Then divide that number by the total number of games played to get the performance rating:
64 / 58 = 1.103 performance rating
This mean that this player won about 1.1 times the number of games that he would have been expected to win, all other things being equal (or 10% more than expected). 1 will always be average, players above 1 are better than average, players below 1 are below average.

MatmaRex commented
I think it would be better if you posted this as a comment in the ladder/ELO suggestion, hardly anyone checks new suggestions.