The 6 x 45 is simply a .223 Remington necked up to 6mm. There is no fire-forming, you just take fired .223 brass and FL size them in a 6 x 45 die. The 6 x 45 started life several decades ago as a bench-rest round. I am quite fond of it and have chambered two bolt-action rifles as well as my AR's in it. It's been quite popular with predator hunters that want more knock-down power than the .223. Over the years I have built dozens of AR's on the 6 x 45 and have yet to have anyone not LOVE them.
The 6 x 45 will fall short of the 6mm AR and the 6mm WOA, but I think when you look at what you have to go through to get that little extra, you will find it's a case of "diminished return". The dies for the 6 x 45 are about 40 bucks, the dies for either of the others are going to run you $100-$150. You will need special mags with the others, as well as a different bolt, and expensive brass. The brass and magazines for the 6 x 45 are almost FREE!
I am building one of the 6mm WOA's, but more from professional curiosity than anything else. I have the ballistics for it at work, if you are interested. I think if the 6 x 45 isn't enough, you should jump to the .243 WSSM and get some real performance.
There is a very fine line between "Hobby" and "Mental illness"