GMAT Strategy

Data sufficiency has always been my weak point because I don't really understand the theory very well. Many of the questions such as if "n^2-4 has remainder 1, what's the remainder for n/3 etc" I have to plug in a lot of numbers to arrive at a choice.

I've done relatively well on my practice exams by plugging in numbers but because yesterday I bombed my real gmat because I spent too much time plugging in numbers. Now I'm determined to learn the material the right way.

To those of you that have aced data sufficiency, are you able to get the answer just by looking at the statements and doing some simple manipulation or do you actually have to plug in numbers? I really wanna study the "theory" behind all these questions so every question should come to me as "is 1+1=2"?

Well, I suppose it means what your definition of "superhigh" is. I'm not a quant jock by any stretch of the imagination, but I am a consistent 49 or 50 on quant (I let all the engineers and science kids get the 51s...). I definitely plug in numbers, although not as much as I used to. I did it on almost every problem with variables when I first started studying (low 700s). Now I try to do things with number properties first, then I see if there's any quick algebraic way that jumps out at me, and if not, then I go to plugging in.

I know some quant monsters will disagree, but I think plugging in is a perfectly acceptable way to solve most problems, particularly if you're NOT shooting for a ridiculous quant score. You just have to be fast with it and flexible with your approach.

I have never done it before but I don't see why it can not be an acceptable approach, IF you have the time. I wouldn't recommend it as a general method to solve the DS problems though.

Yes, like Spaceman Spiff & Jim said, 'plug in' is perfectly acceptable! ... As long as you get answers correct within time frame allotted, any method is acceptable.
But to answer your question:
Do high scorers ever plug in numbers?
Yes of course, they do. BUT NOT ALL THE TIME. Whenever you're certain about the calculations, avoid scratch pad; because working each question on scratch pad and cross checking the answer by plugging in is perhaps practically impossible. (and you've experienced it first-hand?).
Learn more about "number properties" and your problem solving (+DS) speed will increase.

DS is tricky section and need more attention while reading/understanding the problem. Usually you dont have to 'calculate & get answers' in DS questions; you just need to know whether given info is sufficient or not. Earlier when I was new to GMAT (esp. to DS) I used to calculate the answers. ( I know very bad and tempting habit of an engineer! but we need to overcome to that by not spending time on solving sums ...they are lot difficult and are not expected to be solved.)