Same meaning said in different ways.

...

No. "I've just got a good idea" means either a) you have a good idea and nothing else, or b) you have just been given or found a good idea - maybe somebody told it to you, or you read it in a book. "I've just come up with a good idea" means: c) a good idea has occurred to you very recently. It is a new idea, and it is yours, i.e. you didn't get it from anybody else. Occasionally, if the context is clear (i.e. you are known to have been searching for ideas) it means the same as b), above, but this is quite unusual. Usually, to say c), which I think is the meaning you are trying to convey, you would say "I have just had a good idea'.