Should I submit my revised workshop paper to a journal if I don't think it will meet the "30% new material" requirement?
From an ethical point of view, submitting your paper is perfectly fine if you make clear what the changes to the workshop version are that make the paper contain something new.

As far as the question whether you should do it is concerned: If you cannot even convince yourself that you have enough new stuff in the paper to justify (another) journal publication, then it will be insanely hard for you to convince the reviewers that there is enough new stuff. So the submission is likely to waste your time (and possible the reviewer's time -- if the paper gets past the editor) as it is too likely to be rejected. If you are very lucky, some reviewer will suggest a possible point of extension that will make the paper strong enough for another submission.