If superheat is constant across the whole range, then your pressure is not effecting the TEV performance.
The coils are freezing because you have more duty from from your refrigeration and less load, (lower moisture removal) The combination will will allow the suction to drop well below 0C, frost will build, whence ice is built the suction just continues to fall making more and more ice. Id SST were to remain the same, you would get around 15-20% more cooling, but your indoor coil is the limiting factor, so evap temp must drop, this does cause balance out some of the gains made, by dropping the head pressure, so you could be getting around 8% actual increase. You need to check your dew point of the air entering the indoor, I suspect this lower than design.
If this is a major, you could look at fan speed, or sporlan head control system or just a simple hot gas injection to keep the evap above freezing.