Ok so as far as the 3 groove story I have two builds with the 3 groove and couldn't be more happy with them both 1 being in 280AI shooting 168 Berger Hunting VLD 63.5 grns . of Retumbo, CCI250, Nosler brass the other is a 338/06 AI havent had time for working up a good load too many other projects but from the shooting I have done good results. The 300 Rum. is the cartridge I'll be using this year for Elk nad Mulies. so I have a load of 95 grains Retumbo a Bob Cartarucio 211.3 grn bullet Rem. brass and CCI250 going a measured 3150 fps. out of A 26"tube giving me .396" groups col is 3.667" so I can't see why you souldn't get that type of velocity or more out of your 27" & 28" tubes.
IMR 7828 is another good powder for these heavier 30 cal. bullets and I almost forgot one thing this out of a stock 300 RUM. barrel! Just my .02 worth of info.