No need for it to be slower than the .338's 250gr., none at all.
People think of it as a .35 Whelen opened up to .366. Not so.

It is the equivalence of a longer bodied, .35 Whelen IMP. My cases measure 78gr. capacity to the lip in Sako brass and 80gr., water capacity using formed WW '06 brass.
It has the same .454" shoulder, as the Ackley Improved cases.
There is no reason not to load it to it's potential.