I'll try to answer your second question first. I personally don't think that you should re-do the OG. This is because you might remember some of the questions and their answers, so your hit rate won't be that representative of your skill, particularly if you've only just finished the OG. Besides, we tend to memorize especially those questions that we got wrong, so that might also impact your performance: you'd pick a good answer instead of what you'd usually pick just because that question was particularly tricky and you spent a lot of time analyzing it.

I loved the Powerscore CR Bible: it really opened my eyes about many of the subtleties of the CR section. I can't really say if it's going to help your timing, though. I didn't care about that aspect, since my timing was pretty good to start with. It was my accuracy that I needed to work on...

As for MGMAT SC guide, I've heard only positive reviews about it. If you think SC is an area to work on, you should heed the advice of the many and invest in this tool.

Best of luck!