I can speak from some experience on this topic.
About 11 years ago my wife decided she wanted breast implants, so in order to help her decide on the size bought her 2 sets of silicone breastforms from, appropriately enough, thebreastformstore.com so she could take larger busts for a "test drive" before taking the plunge.
Upon advice from the site owner bought her "Transfer Classic II" types because they are more concave in the back to accommodate her natural 34 B cup breasts.
First ordered size 4 forms, which are about 350CCs each and took her 34B to 34D.
Then got size 5s at ~500ccs each, which filled out a 34DDD bra.
As a bonus they came with separate, silicone nipples for extra fun.
She wore them out and about a few times in various outfits to see how she felt sporting such larger breasts in public, and of course we had some hot fun in the bedroom which cemented her decision to get implants.
She ended up going for 450CC implants ("teardrop" shaped silicone silicone, under the muscle, inserted through the areola) which she was, and to this day is, VERY happy with. With these she now wears either a 34DD or 34DDD, but man when she was at peak size right after giving birth (see avatar) and filled out 34G/H bras (without breastforms) I was in heaven...
Lots of pics of her post-enhancement breasts in this thread if interested:
https://titsintops.com/phpBB2/index.php?threads/my-pregnant-wifes-tits-are-getting-bigger.79492/
Even after getting implants she continues to occasionally make good use the silicone breastforms (usually the 500ccs, but sometime doubling up both sets for 850CC extra in each cup) for what we call "fantasy night", with 34G-I bras that I usually buy off Ebay.
Just this summer the surface of the larger set of 11 year old breastforms started to crack a little. They're still usable but I bought her a brand new set off Ebay from China, this time bumping up to 600cc each.
https://www.ebay.com/itm/Silicone-Concave-Breast-Forms-1-Pair-Mastectomy-Prosthesis-Boobs-Crossdresser-TV/274277499372?ssPageName=STRK:MEBIDX:IT&var=574390547162&_trksid=p2057872.m2749.l2649
These are pretty damn good forms! The edges aren't tapered/feathered quite as much as the more expensive, 11 year old ones so they don't "blend" with her natural flesh under bras quite as well, but have about the same feel and weight of nicely firm breasts and with the sturdier bras necessary to support the total weight of real/enhanced/breastformed mass even without a shirt the look and feel is pretty fucking awesome.
pics from 2009 when she first tried the 350CC breastforms: