550 cc HP round sillicone dual plane mentor, with IMF displacement, is this the right choice? Please help! (Photo)

It looks like you have enjoyed your breast augmentation so far and are contemplating upsizing. I have had three patients this month with the same problem and same proposed solution. The 550cc textured implant maybe your best choice for keeping it in position. Sewing the IMF is only part of the solution, you will need to wear a supportive bra after the procedure, to help avoid skin stretch. Try on Mentor sizers to help you determine the size you prefer, your breast diameter limits the best size implant for you. # dimensional Vectra imaging can help you determine the volume you prefer. Good luck...

Since you implants have already dropped and you are considering larger ones, you may need additional support rather than sutures alone. This would be done with an internal bra using Galaflex mesh or Strattice. It would require an in-person evaluation to say for sure.

"Bottoming out" can be the result of a number of factors.  Over-dissection of the inframammary crease can result in sliding of the implants down the chest wall.   Poor quality elastic fibers can allow migration of the implants.  Submuscular implants can migrate or herniate due to high pressures generated by muscle contraction in some patients.
Correcting these complications requires a careful hands-on examination and history.  Thirty years ago we discovered a few major principles
If muscular pressure is a factor, replace the implant OVER the muscle.
When repairing the inframammary crease or synmastia, do not increase the size of the implants.
We originally repaired these by imbrication, but mesh is now available to less experienced surgeons.
Thirty years ago we learned and applied these principles to a severely deformed case which is available for educational purposes on Youtube    breast augmentation repairing pole dancing deformity Bad Boobs !

Since you already have fold issues, you are probably best of not putting in larger,heavier implants. This may cause early descent and failure of your capsule repair.   When treating this issue, I usually use smaller implants or a similar size to the one that was originally placed.

I am sorry to hear about/see the problems you are having after breast augmentation surgery. I think your concerns are appropriate; your breast implants do seem to sit quite low on your chest wall, consistent with breast implant displacement ("bottoming out”).
It is possible that you are feeling the implants significantly along the lower poles of the breasts. Patients in your situation often complain of discomfort in the area of implant malposition. Also, consistent with the breast implant positioning problem, the nipple areola complexes seem to be sitting relatively high, because the breast implants have settled too low. 
Generally, signs of breast implant bottoming out include: 
1. Breast implant "sits" too low on the chest wall.
2.  Excessive palpability or visibility of the breast implant along the lower breast pole.
3.  Nipple/areola complex seems to be sitting relatively high, because the breast implants have settled too low. 
4. Discomfort along the lower breast pole (secondary to pressure from the underlying implant).
5. Relative paucity of upper pole breast implant volume compared to lower pole volume.
6. Increased visibility of infra mammary fold scar (higher on the breast mound).
I think that you will benefit from revisionary breast surgery which will likely involve capsulorraphy ( internal suture repair). Sometimes, depending mainly on the patient's history and physical examination, I will also use additional supportive materials such as acellular dermal matrix or biosynthetic mesh. This procedure serves to reconstruct the lower poles of the breasts and prevent migration of the breast implants too far inferiorly. Associated issues with positioning of nipple/areola complexes and visibility of the inframmammary fold scars should improve with this operation.I find that it is often possible to "go bigger", assuming a well executed capsulorraphy procedure.  In my hands, this is best done utilizing a two layer permanent suture repair. The size of breast implants is then determined based on careful preop communication of patients' goals,  careful intraoperative measurements, and after the use of temporary intraoperative sizers.   The utilization of a larger breast implant helps to achieve the increased upper pole fullness that many patients desire.
 
  Success rates associated with this type of revisionary surgery are very high, assuming  significant surgeon experience, a well executed procedure, and careful attention to postoperative activity limitations.
I hope this helps.