A saline breast implant is designed to have an acceptable fill range or volume. For a 425cc saline breast implant it may be possible to fill the implant to 475cc and stay within the fill range without loss of performance. If the implant is 'overfilled' the silicone rubber envelope will be under increased stress and the life of the implant may be shortened.
Also, the implant will round up and stiffen and lose the soft feel and natural shape. The edges will 'scallop' or form small waves of folds which may be apparent if the breast coverage is thin. The best results will occur when the appropriate implant is selected by the surgeon, and used according to how the implant was designed.
Saline implants are a wonderful product and work very well when matched to the right patient. You can have a smaller concealed scar, avoid leak worries, reduce breast capsules. When breast cover becomes thin, or the implant required becomes large, perhaps 450cc and over, a silicone gel implant may be a better choice.
Spend plenty of time considering choices and trade-offs before settling, and don't expect the implant you tuck in your bra to look and feel like your actual augmentation result. It would be nice, but implant selection is not that easy. Never underestimate skill and judgment.
Best of luck,