I wouldn't really recommend this. While there are lots of practice questions thanks to the Official Guide, there are only so many good practice tests out there, and you don't want to waste them.
I would agree that it's ok to take a diagnostic cold, but it can also unnecessarily freak you out, especially if you haven't done math in a while. If you get all stressed out before you even start practicing, your anxiety and sense of impending doom can really psyche you out and keep you from doing your best.
Rather than trying to brute-force your way into improving your timing, learn the test-taking techniques that will save you time. Things like identifying trigger words in RC passages, making educated guesses through process of elimination, etc. If you review the techniques before doing too many questions, you will spend less time trying to figure the techniques out for yourself and will be a lot more productive (not to mention less frustrated).
Don't make it any harder on yourself than it already is!