The type with no small hole in the plate of the t-stat. He said that without it, the t-stat will not allow a small amount of flow, so will not be surrounded by coolant (sort of vapour locked) and wont sens temp correctly to open.

I have to disagree on this one as I have the original Wahler in my B20A. This one does not have the little hole and it has never ever given me any problems. It shouldn't because if correctly installed, the part of the t-stat that senses temperature and opens it accordingly is fully immersed in coolant on the hot side of the t-stat anyway. If it is not fully immersed, than the cooling system is not filled correctly.

I have to add, though, that I'm speaking about the cooling system with separate expansion tank. The beauty of this system is that excess coolant will be fed back to the system as it cools. That way the fluid-level should always be correct.