The returning oil through the case and intermediate shaft assembly is the largest restriction. Factory came with 1/2" line and 1/2" cooler fittings so I always recommend -8 line size. FYI, the case is 1/4" NPT, not 3/8" NPT. A standard aluminum 1/4" NPT to -8 AN or JIC fitting has a smaller ID through the 1/4" NPT portion of the fitting than a factory brass fitting with inverted flares. So I always recommend steel 1/4" NPT to -8AN fitting and drill it out (I don't like to drill out aluminum fittings), then your case fitting won't be the restriction, it will then become the intermediate shaft and lube circuit components.
All you are gaining IMO with -10 vs -8 is a little more fluid capacity, the -8 line is not a restriction so going larger does not significantly change the flow rate but it is larger so it holds a smidge more total fluid.