well what about giving an XP boost for winners depending on your start place?
off the top of my head, (you may need to balance the rewards)
maybe you get 50% of the xp for 2nd place, and 25% for 3rd
start first- 0 points
second - 1 point
.
7th - 7 points
8th - 8 points
plus 1 point for each starter
eg in a 5 player game , if you start first and win you get 0+5 = 5 points extra xp
in an 8 player game if you start 7th and win , you get 7+8 = 15 extra xp points
in a 6 player game starting last and coming 2nd, you get 6+6 = 12 (but 50% for coming 2nd) so only 6
This would also reward xp for winning games with many players