You're covering the entire CPU with thermal compound before putting the block on? That makes me nervous already. Check
THIS out for a good starting point for applying thermal compound. Bottom line, for most thermal compounds, use very little, and let the heatsink/waterblock do the spreading for you.
You need some pressure for the waterblock to work right. You've got a pretty solid backplate with that block, so you don't have to fear warping the motherboard unless you REALLY overdo it (i.e, tighten the screws until the springs are fully compressed, and then keep tightening some more).