You might lose 100 fps with handloads... Which is negligable considering the gains that come along with the .300 WinMag, like better barrel life, cheaper and easier to find brass and ammo, less powder consumption, less muzzle blast, won't need a muzzle brake (unless you just want one).
Here's a comparison of velocities... The .300 RUM ammo is only 24 fps faster than the .300 WM.
HSM Trophy Gold Ammo 300 Winchester Mag 185 Grain Berger Hunting VLD
HSM Trophy Gold Ammo 300 Remington Ultra Mag 185 Grain Berger Hunting
I'd personally go for an older Sendero in .300 WM over a new Sendero in .300 RUM. But that's just me. Mainly because I don't care for the RUM calibers, the newer rifles have the X-Crap triggers, and the Sendero SFII rifles have an uncomfortable stock that has dual palmswells, whereas the older Senderos have NO palmswells, and are much more comfortable to me.