Uplift with under the muscle 265cc implants. Crossfit concern - should I reconsider the implants?

In general, over muscle placement is the most reliable way to avoid issues like animation deformity and muscle weakening from under muscle placement, especially with dual plane. My preferred way for over muscle is subfascial, which adds a little it of support. It may not be ideal if your body fat is extremely low, but should definitely be considered.

I would strongly recommend having that conversation with your plastic surgeon, as your concerns are certainly valid. The vast majority of the breast augmentations that I do are 'under the muscle.' For patients who are very active (e.g. bodybuilders, CrossFitters), I think it's worth considering going 'above the muscle' for a couple of reasons.
Anecdotally, patients who have had implants put 'under the muscle' have told me that they feel like they have about 90-90% of the strength of their pectoralis muscle after they are all healed, compared to before surgery. For the average patient or for someone who works out sparingly, losing 5-10% of their pectoralis muscle strength may be inconsequential. For those patients who are very active, that 5-10% loss of strength may be a big deal.
Another thing to consider with patients who use their chest muscles a lot is the possibility of developing "animation deformity." When the pectoralis major muscle contracts, it can cause the implant to move in position, which results in a visible distortion of the breast. This can happen in any patient where the implant is 'under the muscle,' but it might be even more noticeable in patients with more robust pectoralis major muscles. If the implant is 'above the muscle,' contraction of the pectoralis major muscle shouldn't change the position of the implant as significantly.
There are good arguments in favor of placing implants in either position, so I would encourage you to talk with your plastic surgeon to make sure that you're comfortable with the route you choose.

In my experience lift with implants under the muscle has not affected physical abilities. Dr. Ted Eisenberg, Board Certified Plastic Surgeon and Author. RealSelf Distinguished Hall of Fame Inductee. Philadelphia, Pa., USA

When undergoing breast augmentation there are a number of choices which need to be made: saline or silicone? Volume: Larger or smaller? Incision? However, one of the most commonly debated choices is that of implants placement: subglandular/ submammary vs. subpectoral/ submuscular? While many surgeons recommend submuscular placement there are distinct differences to each approach. Subglandular Augmentation (“overs”): • Subglandular augmentation means place of the implant underneath the breast tissue but above the pectoralis muscle. Subglandular placement spares the pectoralis muscle which leads to reduced post operative pain/discomfort and no impact on muscle function post augmentation. Recovery is also faster when compared to subpectoral augmentation. • Subglandular augmentation can impact mammographic evaluation of the breast. However, as dedicated breast radiography has become more prevalent this has become less of an issue. Fellowship trained radiologists have become familiar with evaluating breasts post augmentation. It is also important to note that implant position does not interfere with visualization of breast tissue via contrast enhanced MRI (the most sensitive and specific study available for breast cancer detection). • Studies suggest there is an increased risk of capsular contracture when implants are placed in a subglandular space. • Aesthetically, implants placed superficial to the pectoralis major create a rounded, convex appearing breast profile. This effect is camouflaged, at least initially in larger breasted patients. However, as a woman ages fat atrophies and breast tissue descends. The result is a more noticeable implant specifically in the upper pole. Similarly, patients who have thin coverage superiorly are more likely to be able to perceive the implants and at higher risk of visible rippling. Subpectoral Augmentation (“unders”): • Subpectoral augmentation is technically a bit of a misnomer. Traditionally, subpectoral augmentation involves the release of the pecotralis major muscle from its lower attachments. This allows the muscle to “window-shade.” The upper hemisphere of the implant sits underneath the muscle (dual plane). This release contributes much of the discomfort encountered postoperatively by patients. • Subpectoral implants have a lower rate of capsular contracture. • Aesthetically, in contrast to submammary implants (which are prominent in the upper pole- especially in thinner patients), the pectoralis muscle both conceals the underlying implant and flattens the upper pole. This flattening effect creates a natural sloping as one proceeds from the upper portion of the implant to the lower portion. • The most commonly cited drawback to sub-muscular augmentation is the animation deformity associated with contraction of the overlying muscle. Each approach has both costs and benefits. With regards to your specific question, submuscular/dual plane augmentation does impact pec muscle function. There is a real but minimal loss of strength post operatively. Most don't notice the diminished strength. Physically active individuals can return to exercise at 4 weeks but they may notes some losses in strength. However, one must decide if the improvement aesthetically is worth the sacrifice in strength.