I think we should take it back to 2nd order. At least add the ability to interpolate differently for variance and flux quantities. 

My understanding is that it used to be linear in snfit at some time, but I may be mistaken. In SNANA, the interpolation done in doing integration over the transmission is linear. But it has the ability to precompute finer spaced SEDs through a 2nd order interpolation, after which a linear interpolation is not bad. 

It does not seem to be known that 2nd order interpolation is "bad", and time lost apart is probably not bad to have. Pre-computed finer binning by higher order interpolation may be a good idea if we want to save  time.