Re: chronograph readings and eating a litle crow.
Yep, the h@h and 300 wsm are basically the same case capacity. so you'll see more difference between barrels than the two cartridges as far as speed. One of the rifle mags even did a write up on the h@h and wsm IN THE SAME BARREL a few years ago; they lopped of roughly an inch and rechambered so the throat was the same. Net result was dang near no difference.
The 300 rum is more cal than the standard mags, but she thumps you more, which is a consideration to some. She'll also tear up a pipe slightly faster and is more expensive to shoot (but still cheaper than proprietary). 3300 fps with a 180 with mid-loads is a heck of a lot more than 3100-3150 fps wound out with a 180 though.