Is a 255 Gram Silicone Implant More or Less Volume Than a 255cc Saline?

Although your question sounds very simple, it is, instead, very complex. First, none of the now standard implants approved by the FDA are measured in grams. That being said, there are several other problems with your question.
First, although you can get a general idea of what size you will be by using sizers externally, there is actually no way to guarantee that you will be even close to that size after surgery using the same implant size. Several things go into the apparent size you are after surgery. One is whether the implant is over or under the muscle. One under the muscle generally has to be slightly larger to appear the same size as one over the muscle.
Secondly, your specific chest and breast size and width will change the appearance of whatever implant is used. Dimensional Planning uses your size to select the base diameter of the implant. Once this is done, the projection can be selected. With the same base size, the different projections have different sizes.
The short answer, then, is to use cc’s to select about what you would like to be, and then discuss this and any other wishes thoroughly with your Plastic Surgeon so he oe she can take all these things into consideration to select the implant that will best meet your desires.

I am not aware of the use of grams in implant sizing for many years. The old Replicon polyurethane coated silicone implants used to be sized in grams as I recall but these have been off the market since around 1989. If you are having gels used, then have gels used as sizers in a garment to approximate the volume and look you want. I also rely primarily on dimensional measurements and profile preferences to determine what implants I want to use but find that the visual aid of an implant in a garment is very helpful for the patient. Just remember that the implant will look bigger ON you than it will IN you so this needs to be taken into account in sizing in a garment.