How Much Difference Will it Be to Go From 250cc to 375-400cc.

Enhancing the implant volume increases cup size.    It depends on the shape of the patient and breast to know how much cup size will increase.  For many patients of average height and weight, 175 - 200 cc of implant volume will correspond to a cup size.  This may be less for petite patients and more for broader chested patients.  If you already have implants and really like them, it really requires some thought if it is worth it to have another surgical procedure to further enhance them.

Your frame is small and a “C” is a good size.  Going bigger will increase the appearance of gaining weight, and may detract from your hips, and create a little imbalance in size.  Balance and symmetry is what we strive for, and making the entire shape believable, and not an “over surgical” look.  Discuss this with your Board Certified Plastic Surgeon and remember that more surgery increases your chance of scar tissue and capsule contractures.

Unfortunately, it is difficult to answer question because “is it worth it” and “will it be noticeable” are very subjective patient dependent questions.

It is very important to communicate your size goals with your surgeon.  In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as “natural” or “C cup” or "fake looking" means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup  size may also be inaccurate.
I use  intraoperative sizers and place the patient in the upright position to evaluate breast size. Use of these sizers also allow me to select the press implant profile (low, moderate, moderate plus, high-profile) that would most likely achieve the patient's goals. The patient's goal pictures are hanging on the wall, and allow for direct comparison.
I have found that this system is very helpful in improving the chances of achieving the patient's goals as consistently as possible.
By the way, the most common regret after this operation, is “I wish I was bigger”.
I hope this helps.