When the power gauge is full but not yet in use (left hand outstretched animation) does Oni move faster or is it my imagination? Haven't had a chance to test it properly with friends yet.
No. The only time oni can be faster than 115% without perks is with his demon dash
According to the official wiki, the only time the Oni changes speed is when he slows down while collecting orbs or speeds up while using Demon Dash.
So it's probably your imagination - perhaps he seems faster because of the animation changes, or simply because of the contrast between the slower absorption speed and the normal speed you return to after you've finished all your absorbing.
No, he does not move faster when you're fully charged for Yamaokas Wrath... it's just your imagination.
He does however slow down to absorb the blood orbs like Fibijean has said above OR he speeds up when he's in his Demon Dash... otherwise no he does not gain a significant speed boost when his charge meter is full and ready for use.