XL Breast implants, 2500cc to 3500cc. How much would they weigh?

Sounds like you are going down a road that is not recomended by any rational board certified plastic surgeon. That being said the increase in weight would be 4.4 pounds at least, the total would be 15.4 pounds for both implants. That being said I have only seen this size implants once in a woman who had a gig in NYC as a dominatrix, her implant was exposed, infected and I urged her to remove the implants ASAP. She had to do her gig and just wanted an occlusive dressing. I saw her when on call at the hospital. In addition to exposure of the implant, infection and sepsis that could lead to death, back pain , shoulder pain and neck pain can be expected from the weight of such huge implants. THIS IS NOT ADVISED!!!!

In my opinion - these size implants- are absurd. And before you do anything - I suggest that you seek a full and complete psychiatric evaluation. This is not normal.

There are a few plastic surgeons that specialize in XL sizes.I have specialized in XL breast implants for many years.  I have special arrangements for my out of town patients and on average they stay about 4 days but I encourage them to stay longer.
I suggest that you go to XLbreastimplantsdotinfo for my website specifically on this subject. I also provide my patients with other options depending on what is their ultimate goal size. I also provide free FACETIME/SKYPE consultations booked through my Newport Beach offices. 
The silicone XL sizes definitely feel and look more natural but my patients are also very happy with the ovefilled saline impants to XL size or in your case, you may want to switch to a XL custom tissue expander.
 I also provide overseas B-lite silicone implants which also are available to me overseas. These are 30% lighter than standard silicone implants but so far I am only able to obtain them for my patients up to a 920cc size. It is definitely better to go to a plastic surgeon who specializes in XL breast implants for the potential for problems is greater.