How common is muscle deformity after breast implant removal?

When an implant is placed under the muscle (the pectoralis muscle of the chest wall) the muscle naturally has to stretch to accommodate the volume of an implant under it. Some people do get abnormalities with muscle movement such as folds or jumping up of the implant with contraction. When an implant is removed that muscle has to shrink and stick back down to the chest wall. Usually, a surgeon will place a drain into the open cavity where the implant was taken out of to help collapse that space. Even after only a few days with a drain in place and external compression, the muscle will stick back down appropriately. It is not usually necessary to close down that space with stitches unless there was a problem with muscle position to start with. Since you had no issues with the implant in, you are much less likely to have a deformity when the implant is removed. Your skin, however is unlikely to be that forgiving. Do expect that you will have some drooping after an implant of that size comes out. Many patients consider a breast lift at the same time to improve the shape of the breast (that also could be done later if you are unhappy with the final shape).

If you are having animation deformity now, it can definitely persist after implant removal unless it is re-attached, and can become noticeable even if you don't see it now. Another alternative is to keep the implants but convert to the split muscle plane, which keeps muscle coverage over the upper part off the implant where it is needed most but re-attaches the portion that was cut, behind the implant. 

If you do not have animation issues now, you are likely not going to after removal. Most surgeons do not reattach the muscle down to the chest wall. There is often nothing but rib periosteum that you could possibly sew it to, and the sutures would likely pull out.

I have not seen patients have a decrease in performance. Usually patients have felt better.  This has been particularly noticeable in swimmers.

It is not standard procedure to suture the muscle back to the chest wall after explant surgery.  The sutures would most likely pull away.  More important for a nice recovery is to wear a good compression bra for several weeks following surgery.