I imagine that Nosler doesn't come out with their own line of loaded ammo because they are being used by most of the major ammo makers. If they brought out their own line the big companies might find another source of premium bullets. Also, they are selling a ton of bullets as it is. They may be happy the way things are.
In answer to your second question, it depends on what you mean by performance. If you are talking about velocity you can probably beat most of the ammo out there by 5-10% if you try several brands of powder. The big exception would be Hornady's Light Magnum ammo. It is very fast and almost impossible to improve on without going into unsafe pressure levels in your rifle.
If you are looking for better accuracy it is very easy to get better results with handloads. Just find a bullet you like, load it into quality cases with a suitable charge of powder, and try different seating depths until you find one that produces small groups. (This assumes you have a fairly accurate rifle to begin with) You may have to try more than one weight of bullet to find one your rifle prefers.
If you want more info than I can give you here get a reloading manual from Lyman, Lee, Hornady or others and read up on different ways to improve accuracy. Whatever you do, if you try handloading get at least one manual and refer to it often as you get started. It also helps to talk to people at your local gunstore and ask questions. Somebody will probably volunteer to help you get started. I know that is what happened for me. If you try it, good luck!