I can probably add a bit of "sizing" experience. If your girlfriend had a natural B cup worth of breast tissue, and 750cc implants took her to a DD cup, you can guess at the logarithmic progression and extrapolate it. I would guess that of that 750cc, 150cc caused B to C, the next 250cc caused C to D, and the last 350cc caused D to DD, since cup size is a measure of projection but implant volume must increase in height, width and projection at the same time.
Based on this progression, she'd need somewhere around another 450cc to achieve a DDD(F), and yet another 550cc to achieve a FF cup, total of 750+450+550 = 1750cc. Obviously high profile vs regular implants alter this progression by adding more projection and less additional height and width per unit of increase in cc's, which would certainly jive with your assessment of 1600cc high profile implants.
Another factor that needs to be kept in mind is that human breast skin is not going to be happy about being stretched immediately from 750cc to 1600-1750cc. The "comfortable" limit for a "trade-in" is +40% probably somewhere around 1200cc implants. Fortunately, this size of implant (actually a 1000cc rating with 20% overfill) is readily and inexpensively available from Silimed. You can find more info at
http://www.silimed.com . You will need to find a plastic surgeon who participates in the Silimed test program though, but the Silimed folks can help steer you to one in your area.
The good part about going with 1200cc implants is that the "comfortable" 40% skin stretch limit for her NEXT set of implants is now raised to the 2000cc ballpark!