I am just working on a series of posts on using the LSAT to study for the GMAT so I am in a good position to reply to your question.
Unlike critical reasoning LSAT problems, which can be a little bit "hit and miss" as in some are great to study others are not and it is hard to tell the difference sometimes - LSAT Reading Comp is good to study for the GMAT.
The real problem with most reading comp that is not part of an official test, you mentioned some unofficial sources in your posting, these are usually not edited as well as official questions that have appeared on the LSAT or the GMAT. So using LSAT questions is good because they are very well-written and edited.
As you mentioned LSAT passages are generally longer and the questions can be tougher. This is not a problem, so long as it does not frustrate you. I will say that LSAT questions are not that much tougher given the time standards on the GMAT. Much of what makes the LSAT so tough is the short amount of time. Only 35 minutes for 28 questions! When you practice 28 LSAT questions, give yourself the standard 2 minutes per question so you get 56 minutes instead of 35!
There is one type of LSAT question that is more recent that does not translate to the GMAT and that is the comparative passages, which has two reading passages on similar topics. To avoid this you can go to any LSAT tests before 2007. You might as well get some from the late 1990s or early 2000s as these will be cheap. Probably 5- 10$ for used copies. The books are the "10 official LSAT" series and there are several books.
In short, I do recommend using the LSAT over the other sources you have mentioned - just as long as you have studied the GMAT first! For that I of course recommend the official guide - but also some type of a strategy guide, such as the Veritas Reading Comprehension book. Once you know what to do with GMAT passages and what they look like studying from the LSAT should help.
Good luck!
