Do drivers increase low end card's performance significantly as well?

Might as well stay updated.

No reason not to keep your drivers updated (except in the rare occasions where either side royally screws up).

they might or might not. you also shouldn't trust nvidia's numbers. They are often greatly exaggerated. Their 25% increases are often closer to 0.25%.

Maybe my question wasn't clear enough. Why doesn't nvidia post performance boost for all of its latest GPU series e.g 600 series...why does it only mention a performane boost in gtx 680 and gtx 660

Drivers are usually optimized for high end cards. That's where the money is. Low end cards don't benefit as much but it's still good to keep up-to-date.

Actually most profits are made in the middle to lower middle end segment.

only people who buy high end cards really care about absolute performance.

Because the 680 is the high end and the 660 mid-range, they are demonstrating roughly the performance boost you can expect with your hardware.
Plus they know if they make a big enough change, review sites will be all over it and provide a heap more detail that will be trusted more than their statements.

Also they can assume that people with a 630m in their laptops aren't going to be overly concerned about gaming performance, so why use up headline space for a small market?

Now that is just being blatantly ignorant... Some people "care about absolute performance" but don't have the money for high end cards and that's why you see people overclocking middle and low end cards.

How many people are OCing 650ti and below?

Hah! I remember reading a site once where they had a section for extreme low end overclocks.

I have personally seen people (including me) overclock GeForce 2 GTS's, 4 MX's, FX 5200 Ultra's, 6600's, 6600 GT's, 7600 GT's/GS's, 8500 GT's, 9500 GT's, GTS 250's, and GTS 450's among others.

THe place where i live, i bought this laptop for 999 CHF which is like 1100$ and it has gt 630m so i don't think that it is always about nvidia only providing support to higher end cards because they are expensive and people who buy them should be supported for it...

I just wanted to know if someone experienced a noticeable difference in performance when jumping from older drivers to newer ones on their "low end" cards...I mean, it's obvious that there should be improvements but are they significant enough?

Well considering the minimal effort involved in updating the drivers and lack of any negative factors, dont see how even a minimal performance boost wouldn't be worth it.
Just update the drivers, theres no reason not too. If you really want to see the performance difference before and after, run 3D Mark Basic before and after updating. WIll literally give you a number which indicates your graphics performance.