I don't know what your problem is but. If you have 15 degrees of subcooling but high superheat, the first thing to realise is. The valve isn't opening up. Not pinching down. If she was wide open, you'd see lower superheat. Your stacking liquid.
A subcooling measurement is an indication of quantity of refrigerant charge. The reason for it is. We need to insure a full column of liquid to TXV over all load conditions. As she throttles to it's low point, raisng superheat, subcooling begins to rise, as she opens, subcooling should lower. If we had little subcooling when the TXV is at it's low point, and then she opens up to react to it's load, we may not have enough liquid sitting in receiver for her to eat and she'll begin eating vapor which is not good. That is why the receiver level is allowed to go up and down, to accomadate the Throttle of TXV with a constant feed of good liquid yum yum.
As far as too much at 15 degrees of subcooling, man, you got to tell me what everything is first before I say it's too much. I got some of my larger systems running 30 degrees of subcooling, during 20% loads.
A warm box, a cold box, a luke warm fuzzy box. A properly adjusted balanced port TXV will start to settle down almost as fast as you put the system to "on". A valve is set to a range, and she will throttle with in that range if shes working right. Superheat is superheat. Just cause the box is warm doesn't mean you should have abnormally excessive superheat. It will just be in a higher temp range.
Cap tubes are different. A warm box will cause high superheat, until temp begins to drop.
Sorry guys...I kinda dropped the ball on the updates. I went back out earlier this week. Box sitting at -15*, 8* superheat at bulb (hunting as high as 12 at times) and about 15* at the compressor. Subcooling up to 22*. Head came down to about 295* (I think that was it) with a 90* ambient.
Everything's looking great! Thanks again for all the info.