It is actually possible to complete Donkey Kong Country Returns to 200% - as if 100% wasn't enough!
To do this, and earn some unlockable images, complete the game as normal, getting every KONG letter and puzzle piece. You then need to complete it as before, but in Mirror Mode! You will then unlock the following images for the gallery:
When you defeat the Boss of the following island sections the corresponding music gallery which features the tracks played in the levels of that particular section become available at the 'Extras' menu. Note: You cannot use Super Kong to get past the Boss fight.
Unlock Beach Music Gallery: Defeat the Boss of the Beach section.
Unlock Cave Music Gallery: Defeat the Boss of the Cave section.
Unlock Cliff Music Gallery: Defeat the Boss of the Cliff section.
Unlock Factory Music Gallery: Defeat the Boss of the Factory section.
Unlock Forest Music Gallery: Defeat the Boss of the Forest section.
Unlock Jungle Music Gallery: Defeat the Boss of the Jungle section.
Unlock Ruins Music Gallery: Defeat the Boss of the Ruins section.
Unlock Volcano Music Gallery: Defeat the Boss of the Volcano section.
When you complete the game the Banana Temple will become unlocked but it will not be accessible unless you have collected the K-O-N-G letters in every world and got the orb in each of the world's temples.
Beat ALL the '-K' levels by getting K O N G on every level in a world and then beat the game. When you accomplish this a golden temple will appear on the world map. When you beat the level there 'Mirror mode' will become available. Mirror mode is a tougher setting to complete because Diddy can't be used, you only get one life, the level is reversed, and you can't use items.
ALL the levels in a world except for the Boss levels have hidden K, O, N, and G letters. When you collect ALL 4 of those letters in ALL levels in a world a bonus level will become unlocked. There are a total of 8 bonus levels that can be unlocked.
In level 2-2 (Sloppy Sands) before the checkpoint pig with the two squid turrets below a bridge there is an area that you can use Diddy Kong's jetpack to super bounce from the lower squid to the higher squid shot out from the turrets. After hitting the higher squid just bounce again on the lower squid. This can be repeated as many times as you want to get the desired amount of DK Coins.