This is an excellent conversion choice. You will gain some ballistic advantage in this conversion and very experienced long range instructors like Todd Hodnett prefer this "medium" magnum cartridge.
Also the US military is strongly considering replacing the M240 7.62 x 51 NATO medium machine gun with General Dynamics' new low recoil medium machine gun chambered in yes, .300 Norma magnum, for an amazing ballistic "overmatch" to Combloc ammo. (GOOGLE it)
General Dynamics chose the .300 NM for its great ballistics and the fact that it runs in a belt fed MG better than the .300 Lapua magnum. Their MG is lighter than even the lightened M240 but, yes, the ammo is 10% heavier. It's a good trade-off for those ridge top-to-ridgetop fights in places like Afghanistan or other "Stans" and alpine areas, not to mention wide open plains.
Still, the .300 Win mag is close, as in horseshoes, etc. I own both hunting and target .300 WM rifles and don't feel at any disadvantage. BUT... if I need to re-barrel the target rifle it will be in .300 NM.
Eric B.