3/8 is fine for ice bath or subzero. When I go sub zero though I think I might do 1/2" for the after chillers and 3/8 for the pre chiller.
As long as with subzero you have the gravity assisted self drainage you should be fine. I'm currently using the regular 3/8 wort chiller from nybrew supply just in an ice bath and have no issue except when I top off my tank and the pressures are higher I get a little clogged but just take the coils out the bath for like 30 seconds and turn em around a few angles and that solves it.
As long as with subzero you have the gravity assisted self drainage you should be fine. I'm currently using the regular 3/8 wort chiller from nybrew supply just in an ice bath and have no issue except when I top off my tank and the pressures are higher I get a little clogged but just take the coils out the bath for like 30 seconds and turn em around a few angles and that solves it.