You may be much more aware of a fuller profile implant
I certainly agree with your surgeon that the implants should be placed under the muscle. Only the lower portion of the muscle will be released with either implant, but the full profile will protrude more. This may make you more aware of the implant when you flex your pec. Most women adapt to this, but not all. A lower profile always looks more natural anyway. Your muscle size will not significantly increase the fullness in the upper pole with either profile. Good luck!
Body builder and implants
There are issues for both pocket placements. Above the muscle may lead to very visible implants if you are very thin, and under the muscle may lead to animation issues when lifting.
Breast implants and power lifting
I usually recommend above muscle breast implant placement for body builders, weight lifters and other athletes. Anatomically shaped breast implants can provide a nice, natural breast augmentation in this setting. Good luck!
In my opinion with your avocation as a power lifter, I would consider a subfascial or subglandular placement of the implants and take the muscle out of the equation. You are at a much higher risk of having the implants displace laterally when the implants are positioned under the muscle.
Without seeing you and examining you it's difficult to give you any real answers. In general, you cannot see 30 mL difference in implant volume. Also, the higher profile implants tend to produce more of a step off from the chest wall to the nipple. This step off becomes more profound the less breast tissue you have. The amount of muscle or its thickness hardly comes into play in this regard. The more important thing for a weightlifter to understand is that the trade-off of the advantages of a sub muscular placement may be outweighed by the animation deformity of pectoralis flexion. If you have enough soft tissue over the muscle you might want to consider a sub glandular placement. It sounds like your plastic surgeon has a great plan. Good luck with your surgery.