Hello. First, I am project lead on the BLACKOUT, so consider I am biased.
The 125 grain velocity Wilson states for the 300 AAC BLACKOUT at 2275 is probably what we will load 125 grain ammo to. It is a nice conservative number that I feel comfortable we can always make and stay within SAAMI specs of 55,000 psi. Can hand-loaders get more if they want to push it to 60,000 psi against SAAMI specs? Sure.
Note that the BLACKOUT has a longer throat, like 6.8 SPC-II does compared to 6.8, or 5.56mm NATO compared to 223, so the difference is less than some might think. There may be a 125 fps difference, but it may also be about 60 fps - at equal pressure. I am just not sure, as to properly compare them one would have to take a 300 AAC BLACKOUT SAAMI pressure test barrel, load ammo to 55,000 psi, test it, and then re-ream the same barrel to 7.62x40, and then load that to 55,000 psi, and then test that.
I can say this - I am certain that 300 AAC BLACKOUT will do 2450 fps with a 110 grain bullet from a 16 inch barrel while at or below 55,000 psi. That we have tested. I have also pushed it to 2600 fps in a 20 inch barrel, but it was probably over SAAMI pressure limits and so I can't recommend that.
Remington has five more loads under development for release this year, and AAC will be distributing ammo from another company soon. I know of about 25 loads for 300 AAC BLACKOUT that will come out this year. Also some very large gun companies are working on products.
Choosing is like picking BluRay vs HD-DVD. I am going to bet on 300 BLK due to over 40 companies already supporting it, and it being more versatile by accepting a wide range of bullet weights. Also people in CA and certain other states cannot get new mags, and the military wants subsonic capability. The military will help make 300 BLK popular, and people will want the same thing they use. Also getting the last drop of power is not everyone's goal, else 10mm would be more popular than 40 S&W.