Just an idea/suggestion...
Add up the prestige ranks of either side, and divide to provide bonus to less ranked side.

Defiants (x) total prestige in team 56
Guardians (y) total prestige in team 48
x/y * 100 = 116.7 (ish)
Guardians get a 16.7% bonus to favor in kills and end of match as underdogs.

However bonus cannot exceed 50% so you dont get more at end of match from losing to a super premade.

If a lower leveled prestige ranked team manage to defeat a higher prestige ranked team then they get bonus for achieving such awesomeness.