Yep- he can be a jerk like that. He is short sighted and thinks he is protecting his product. He should be helping others as it would increase his business. I have spent more money shooting my rifle than the price of the rifle. Who cares that I did not buy the gun from him he should still want me to spend money but guess not.
This should give you an idea of what to expect with subsonics in a case similar to the 30BR:
Sierra 220gr/ 2400 10.2gr/ Win primer
Sierra 220gr/ 5744 11gr/ Win primer
Sierra 220gr/ 4759 11gr/ Win primer
Sierra 220gr/ Trail Boss 8.0gr/ CCI 200 primer
With a 150gr hunting bullet, you should be able to get 2600fps or better with H4198 powder.