You should feel some difference, but it won't be marked until the speeds - and the actual gear - gets lower. In 5th or 6th gear, there is very little effect on speed, but as you get down to 4th and 3rd and 2nd, you will feel it more. Help the PAC by applying the service brakes firmly but briefly to get speed down where the tranny can downshift to a lower gear.
I recommend that you do NOT change the target gear to a higher one. That would defeat the purpose of the PAC brake at lower speeds. The target gear can't be too low, because the transmission will never reach it unless speeds get low enough anyway. By targeting 2nd gear, you get firm brake assist once speeds get down around 20 mph. If the target was 4th gear, you would get no additional brake effect below about 35-40 mph.
It is possible your PAC is inoperative. With the PAC engaged and descending a steep grade, brake down to 35-40 mph and coast a few seconds, then switch the PAC off. You should feel an upshift and an increase in speed. If you do not, have it checked.