Keep in mind, Sgt. Friday, the conditions Santa Fe faced with their proposed Grapevine route of 1915. Much of their increased traffic was due to the Kern County Oilfield boom. Once a pipeline was installed this traffic largely went away. Also, The Octopus threatened to void Santa Fe's lease over Tehachapi the moment they turned a shovelful of dirt on the Grapevine Route. On the engineering side, as you mentioned the proposed route would have required many tunnels on both the North & South ends of the Piru Gorge; many steel viaducts were also planned for the climb up Grapevine Canyon. Of course the trackage in Piru Gorge would be under water now with the construction of Pyramid Dam & Lake. Check out John Signor's Tehachapi book for more details.
To OPB, I agree that current running times on the Coast Line are sad, especially considering the lack of freight traffic. But I must question those who have talked about 6 - 7 hour drive time between Los Angeles & San Francisco. It took me longer than that 15 years ago, and I 've been told traffic has only become worse.