Lots of people use the frame or body for the grd return....& have trouble free operation. Steel has 7 times the resistance of copper. So when the CSA of the steel is 7 times [ or more ] the CSA of a copper cable, the resistance will be the same...or less. The frame or body is not the problem. Poor connections or connections with insufficient contact area will add resistance. Also, starter motors come in different HP ratings. King of the Hill such as the MSD dyna force produce over 3.0 hp.
The fact that it cranks 'like a banshee' when cold suggests the cabling etc is ok. You may have a 'tight' engine that needs a higher HP starter. The starter may also be getting hot, internal copper armature winding increasing resistance from heat...& losing power.
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