No they are not all built the same.  It'll probably open even faster than the the 150 Super X.  There's a reason they have the nickname "Coreloss".  Lots of elk and deer are killed by those bullets every year, but your magnum should be slowed down with heavier bullets if you are going to shoot the cheapest stuff on the shelf and expect good results.  When you refer to game as CXP2 or CXP3, you've obviously spent a little too much time on Winchester's website.  Take a broader look around.  If you want to shoot elk sized game with a magnum rifle that will stress the basic cup and core design, you need to increase your bullet weight to 180 or 200 grains.  Getting that impact velocity down below 2700 fps should help the situation.  There's lots of good stuff out there, but you're asking for trouble when shooting elk with 150 grain basic soft points at breakneck speeds.

If you want lightning fast (meaning 150 grains or lighter), you'll need to go to tougher construction.  Winchester's XP3, the various Barnes TTSX and TSX loadings from Federal or Barnes or my preference: the Nosler Partition (I handload, but Federal loads them too).  However in a Partition, you may still want to up the bullet weight, as it will almost always shed the front core

Try the Hornady Interbond for elk as they are as good as it getsfor penetration/expansion and most rifles shoot them pretty good.

In the past all the CoreLokts I used performed perfectly well, but that was years ago.  I reload my own now and have for several years, but I use Speer bullets for my handloads.  For all the fancy exotic ammo that's out there available today, the CoreLokts have probably taken more N.American biggame than most hunters really care to admit. 

I'm of the opinion that you don't need a premium bonded bullet for deer sized game, not really even for cow elk.  But many will disagree.  I'm just giving input on my own experience with hunting deer, elk, and antelope.

The Partition is a pretty good bullet.  A good old mainstay in premium bullets.  Some will say they are not good, others love them.  I wouldn't say they are any more or any less accurate than others on the market.  However I think the only true way to determine accuracy is to remove the human element out of the equation when sighting-in, meaning a good lead-sled rest and a phneumatically actuated trigger puller. That's just for sighting-in and zeroing, not for actual shooting practice. By doing this you'd be suprised just how accurate most bullets out there will be.  By doing this I've found even factory ammo to be pretty impresively accurate.  Again each rifle and barrel will like some components better than others, even if it is minute.  Again I still maintain that a so-called premium bonded bullet is not nessesary for clean quick kills on deer, pronghorn, and elk. I've used Speer HotCors and Speer Grand Slams for quite a few years and as long as I've done my part those bullets have perfromed wonderfully.  Just haven't been able to find the Grand Slams for a couple years now .

Your talking hunting rifle's. Give up a bit of accuracy for proformance. you have a load shooting an inch and a a quarter or half from the bench off sand bags, your doing fine. I use mostly Hornady Spire Points, not overwhelmed by the custon bullet's. A good killing shot with a proper weight bullet is a good killing shot. The old Speer hot core is about as good as a bullet can get. I shoot Hornadys because the shot a bit better.

Using most anything under 180gr in a 300 mag is looking for trouble. The TSX type bullet's would be the exception. Nosler partitions have a huge following but I've never owned a rifle that liked them very much. The older ones had a habit of eliminating the from of the bullet, shearing off the jacket at the partition.