This matter is really difficult.
On one hand good performance should be rewarded, bad performance... well, not punished, but at least NOT rewarded. On the other hand rewards for killing are absolutely inacceptable, as they will make gameplay hell, everyone will be trying to kill steals on focus on killing rather than winning the round than they already do now. You will be surrounded by asshats, all the time, stealing your kills and/or wounding or killing you in that process. And a solely performance based reward system only increases the gap between top players and new players.
I think you must find something in between, where your performance helps you, but on the other hand where everything stays fair for everybody. I guess kills are a bad base for rewards, as they don't necessarily show how much someone contributed to win the round. And I think different classes should have different goals to achieve and thus should receive rewards for different stuff.
One funny idea I has some time ago was to reward infantry for kills which are made by teammates close to them, but no rewards if you make the kill yourself, and the more players of your side survive, the bigger the percentage of that reward you receive at the end of the round. It could be funny to see infantry actually supporting and trying to defend each other, instead to just autowalk like a lemming wherever you see the closest enemy.
But whatever you do, I think a change of the upkeep system goes hand in hand with a new reward system, so this should be changed, too.