Suggest you check out the following reference: "Pipe Restraints: Design Fallacies" by Patrick J. Creegan, F. ASCE which appeared in the Journal of Transportation Engineering, Vol. 109, No. 6, November, 1983. This peer reviewed paper will give you clear definitive guidance and several...
Suggest you contact UDOT (Utah Dapartment of Transportation) since they have recently installed numerous SPUI's during their reconstruction of I-15 for current data and lessons learned. The planning department would have data on capacity etc.
If one assumes that the SPUI is replacing the traditional "diamond" style of interchange then the difference in capacity would be slight .... however the area required for the SPUI (ie. the "footprint") would be smaller for the SPUI than for the diamond. However this smaller...