Your friend has done a great job of putting doubt about your set up before a shot has been fired. That's a great thing your friend did for you.
There is no doubt that a quality 150grn slug being fired out of a quality rifle like yours will do the job.
You have 7 to 8 months to prove your set up to yourself. Get the doubt out of your mind and go to the range and become confident in yourself and your choice of equipment, do your job and your weapon will do the rest for you.
I have fired the Nosler E-tip out of my 7mm Mag Ruger #1 and they were very accurate. I didn't have an opportunity to take an animal with one but I have no doubt that if I did my job they would do theirs.
Clear your mind, go prove to yourself that your equipment will do what you want it to do and go take the Dall sheep of your dreams.