HLT coil exposure during mash?

[stickyfinger]

I have always filled up my 20G HLT tank to 12.5 gallons, even for a 1.040 no-sparge brew just to ensure that the coils are completely covered. I was wondering if anyone has filled their HLT to like 1/2 coverage of the coils, maybe around 6-7 gallons to make the heat-up time shorter. Will this result in poor temp control for the MLT? That is, is the full length of the HLT coil required to be under the level of the liquid to result in good heat transfer?