we should get extra points for how much time is left on the clock if we manage to kill all 4 worms.... like 10xp per second or something :-)

Yep, I've already suggested this: http://forum.team17.co.uk/showthread.php?t=59849

Since killing all opponent's worms in 3 minuts is somewhat a hard work, I strongly think that getting awarded with extra coins, xp or even a weapon would make worth all the effort you had to kill his/her entire team.

This is a pretty cool idea, will forward it to the designers! However, remember, you already do get extra coins for killing all 4 of the opponents Worms!