The trick to eliminating leading and improving accuracy lies not in using a harder lead bullet. A "soft" bullet of, say, bhn 9-12 will work wonders in those regards, as well as expand reliably if driven 800fps and faster. Size the bullet for a slight interference fit in the cylinder throats - ignore barrel groove diameter - and use a good lube. (Hot powder gasses blasting past the too small bullets is what causes leading in throats, forcing cones, and bores.) I know that going softer is counter intuitive, but a soft bullet will obturate to fill out and seal those gasses in, and a hard bullet won't.
Those Speer lead SWC's are good bullets, but they make them on the small side so as to work in all guns. (Yes, Virginia, there was/is a large discrepancy in throat diameters over the years. It's why I keep .356, .357, and .358 diameter bullet sizing dies handy so as to match my bullets to the particular gun.*) That, plus the minimally efficient dry lube they use, is why the Speers aren't the greatest choice when working up loads at the upper end of the scale.
*I have three .38's I load/have loaded for: a 40's vintage Colt Officer's Model Match took .356's, a 60's vintage S&W K-38 took .358's, and my current 1949-vintage S&W K-38 takes .357's.
These speers are already sized a little small so I'll just have to see how they do in my guns. Would it be worth trying Lee alox over the top of the factory lube? I'm hoping for 900 fps.
Bb
