Should I submit a revised version of my paper before hearing back about the first version?
You should not submit your revised version at this time. If it had been just a day or two since you submitted, it would be a different matter. However, more than six months later, it is quite likely that the reviewers will be angry to receive a new version of the paper, particularly since you have made a great deal of changes. This will be even more so if they are a substantial portion of the way through the review process. (It is also unfair to the reviewers and editors if the author can keep changing and revising the paper while it is under review.)

Your best bet will be to wait until the reviews have been returned, and use your revised version of the paper to "jump-start" the revision process.