5'8", 165 lbs. I am 38C/D, 280cc saline. Can I be a 38DD with 300cc Mentor & lift?

Thank you for the question.  There is no accurate/reliable/predictable correlation between size/profile breast implant utilized and cup size achieved. Despite your good description of goals, online consultants will not be able to provide you with specific enough advice to be truly helpful. Ultimately, careful communication of your goals (in my practice I prefer the use of goal pictures and direct examination/communication in front of a full-length mirror)  will be critical.
Generally speaking, the best online advice I can give to ladies who are considering  revisionary breast augmentation surgery ( regarding breast implant size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully.  Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. Ask to see lots of examples of his/her work.
2. Have a full discussion and communication regarding your desired goals  with your plastic surgeon. This communication will be critical in determining  breast implant size/type/profile will most likely help achieve your goals. 
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 "DD cup” or “a little more projection/fullness” etc means different things to different people and therefore prove unhelpful.
 Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup  size may also be inaccurate.
3.  Once you feel you have communicated your goals clearly,  allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals.  Again, in my practice, this decision is usually made during surgery.
I hope this (and the attached link, dedicated to revisionary breast surgery concerns) helps.

Adding 20cc will hardly be visible. Most people can't see that they are obviously larger until 100cc is added. It usually takes 250-300 cc to add a cup size.

It is obvious that you
want to go significantly larger so I doubt if only adding 300ccs will make
much of a difference. Bra cup size is not accurate and it is impossible to
equate cup size with implant size. What looks best regardless of the size
is what is important.