Travelerbk, I personally agree that the LSAT materials are really good practice for GMAT CR and RC. There are plenty of cosmetic differences (the LSAT is more time pressured, the GMAT doesn't have as many parallel reasoning questions, LSAT passages are sometimes a little bit more abstract, LSAT passages are longer, etc.), but if you're killing the LSAT, you'll probably be more than fine on the GMAT CR and RC. The core skills (precise reading and logic) you need for success are exactly the same.
I do strongly recommend maintaining your skills on CR and RC, though. You'll find the first chunk of questions in the
OG (and official GMAT verbal guide) a little bit easy, but it's wise to continue doing at least one set of CR and RC per week, just to keep your skills sharp. And I agree with Vercules that it doesn't hurt to get some exposure to the GMAT CR and RC questions, since the styles are just a little bit different. Spend an hour or two per week on CR and RC, and you'll do wonderfully.
In all honesty, you're in a great position. CR and RC are extremely hard for some people to improve, so consider yourself fortunate. Sentence correction is definitely the most "improvable" part of the GMAT, and it's mostly a question of ignoring the "sound" of the sentences, and focusing on mechanics, logic, and meaning.
MGMAT's books are probably your best bet for both SC and quant, though you might also check out the Thursdays with Ron videos--a lot of GMAT Club members swear by those. Generally speaking, it's a good idea to focus on building up your knowledge of quant and SC fundamentals first (using
MGMAT books or something similar), so you might want to take a step back and work through the
MGMAT materials before you burn through the entire
OG.
Good luck with everything!