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

example:
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.