There is Total Equivalent Length for elbows and it is given in terms of feet of straight pipe, so an elbow might be equal to 3' of straight pipe in flow resistance. Of course, I lost the link to this. You could try the Engineering Toolbox site.
I suppose the furnace efficiency could be reduced to even lower values so then it becomes "How much more are you paying each month for this unorthodox piping?" and "What are your alternatives?"