Check out this thread... hopefully it will give you the insight you are looking for, but I will also try and answer your questions.
1) Which one works best, high or low? - really it depends on what you already have... if you are using BBQ tanks then it is HP propane
2) With 1/2 SS T-couplers welded on will that work with both high and low pressure systems? - charge it with an air compressor @ 10-20psi and check for leaks, if it is good at that pressure you should be fine.
3) Do you lose efficiency with high pressure or low pressure? - Not that I am aware of, propane has the same BTUs per pound... now if you are comparing propane to natural gas then yes... NG has less BTUs per unit than propane does.
4) What are the recommended burners for each type system? I have what I assume to be a banjo burner on a blichmann floor stand that I've used for years- I'd like to use this in the new build if possible. - I am using a Top Tier (blichmann burners i.e. banjo burners) and they have worked well with both HP propane and natural gas provided you use the correct orifice for the burner.