Your obesity puts you at an increased risk for any kind of surgery but especially for surgery using your own tissue (DIEP or TRAM flaps). Your best bet would likely be bilateral tissue expander reconstruction and silicone gel implants. It is a shorter operation which also lessons your obesity-related risk factor.
Natural Breast Reconstruction After Prophylactic Mastectomy
Prophylactic mastectomy is an excellent option for reducing cancer risk in the BRCA+ or high risk patient. Reconstructive option are based on overall health, body status, anatomy, tissue characteristics and personal values.
In your case, this is an elective procedure. I would recommend that you embark on a healthy weight loss program to reduce your BMI from 40-30 (Weight goal 190 pounds). Being closer to your ideal weight will make you healthier, decrease your anesthesia risks, decrease your risk of complications from surgery, and improve your aesthetic outcomes.
In my practice, I perform mostly implant based reconstruction, which can provide a symmetric, soft, natural result.
Most Natural Breast Reconstruction
You pose an interesting challenge. The reflex response for me is DIEP flaps for breast reconstruction, but due to some of your existing conditions (obesity and abdominal hernia repair) DIEP is not necessarily going to be the most appropriate. I would need some more information be I gave you an opinion that I felt comfortable with. Depending on the location and extent of your hernia repair, you may or may not be a candidate for abdominal tissue harvest. Because of your obesity, DIEP would likely result in more fat necrosis, so if we could use abdominal tissue then a muscle sparing Free TRAM flap would be more appropriate. If we could not use your abdominal tissue because the blood vessels were compromised from your hernia surgery, then I would recommend TUG flaps from your inner thighs. If all those were not ideal then I would fall back on Latissimus flaps with implants. SGAP or IGAP is also a possibility, but I would advise doing one side at a time.
There are many options for breast reconstruction including tissue exapnders and implants, latissimus flaps with implants, fat grafting for small contour problems, TRAM flaps, DIEP flaps, SGAP, and IGAP flaps to name a few. You require an exam first to figrue out what is best for you..
Prophylactic Mastectomy with Immediate Reconstruction
Based on your question, the most natural breast reconstruction is a tissue reconstruction with DIEP or TRAM flaps. Based on your current weight and history of hernia repair with mesh, I would recommend that you consider immediate one stage reconstruction with a skin sparing mastectomy and gel implants or two stage with a tissue expander placement.
This does not burn any bridges. Therefore, if your BMI drops to a safer level and the hernia repair does not interefere, you may be able to consider tissue reconstruction at a later time.
High risk patient seeks natural breast reconstruction
I would strongly suggest that you request your surgical oncologist (the physician performing the prophylactic mastectomy) send you to a PS, with whom (s)he can work and offer you an immediate reconstruction, using either a) tissue expanders or b) one stage "direct to implant" with an Alloderm sling. Irregardless of which option is adopted, your PS should ideally mark your breasts for a "reduction" mammoplasty, so that the excess skin can be removed and the reconstructed breast reshaped to provide a smaller but natural appearing set of breasts. I also agree that unless your surgery is performed at an academic facility which has great interest and experience in autologous (own tissue) reconstruction (i.e. TRAM or DIEP flaps), your results may be suboptimal. Good luck.