Not sure about your second paragraph, Mutt. Looking at the B737 graphs I have for Mass and V1 reductions you seem to get a lower take-off weight as depth of contaminant increases and a higher V1.
I interpret this as braking performance being fairly equally poor whether you have 2mm or 13mm of slush, and therefore the VSTOP vs weight line stays fairly steady, but the impingement drag caused by the greater contaminant depth causing the minimum speed to go, VGO, to increase at any given weight. The effect of a relatively stationary VSTOP line and VGO increasing is to reduce MTOM and increase V1.
Am I using these incorrectly? I'd appreciate the advice.