How to improve in Quant from 45 to around 48?

oh and one more thing, why do you think that Manhattan set and GMATClub problems would not help as much as the Jeff Sacksmann's set?

It was for me. It also cemented some concepts and processes by doing these questions how the Manhattan guides tells you to. I particularly don't like the explanations for rate and two overlapping set problems. I use the methods from Manhattan for these.

You do.

I did not find the set THAT challenging, even the extreme set. Ironic. At times I even thought it was a wast of time. Nonetheless, my score did go up to Q50 right after I finished the set. I don't know what is at work, but reviewing the CAT I do notice my careless errors (i.e.: "that was stupid of me") go way down.

In my opinion they are way too hard. I may be wrong though, maybe harder questions is what you need. Many even suggest GMATClub for 45 to 48. There is a free test from GMATClub, so test it out for yourself.