tough question. generally i would agree with the posters above and say that sweat chloride numbers are not predictive. and certainly i would say that it isn't conclusive in any case. each individual with CF is different and there's a LOT that goes into determining outcome.
all that said, i will go out on a limb here and say that very generally speaking, young children with higher sweat chloride levels who are diagnosed as infants tend to have more severe presentation than people who are diagnosed as adults, who not coincidentally also tend to have lower (often normal) sweat chloride levels. for example, the average age of needing transplant is significantly younger for CFers diagnosed as infants vs. adult diagnosis (according to the data released by my tx center, anyway). keep in mind that back when i was diagnosed (28 years ago) sweat chloride was the only way to diagnose CF, since the gene wasn't discovered until '89. so among my generation of CF adults, higher sweat chloride is the primary differentiation between people who were diagnosed as infants and those diagnosed as adults. and we do tend to have a more severe presentation.
but of course that just means that people with chloride levels high enough to clearly indicate CF might have a more severe presentation than those with normal levels. it doesn't say anything about whether a number of 100 means a less severe presentation than, say, 160. however, since it is the chloride transfer defect that causes the build up of thick mucus, it would make sense to say that the higher the number, the worse off the transfer and thus the higher the possibility of severe symptoms.
honestly though, i wouldn't try to predict the future based on a number, esp. since sweat chloride levels often change with age.