hello there,
I own modelart sandstorm 1:16 car and  currently i am using 800mAh 7.4 V 15C battery. If I upgrade to 7.4v 1300mAh 25C Li-Po Battery; then would that help?
 
what changes can we see in car after we increase the "mAh" or "C' ??
 
this thing always troubles, me; help me out 

The only difference you should find is longer running time, roughly 60% more.

The mAH is the capacity. 1300 divided by 800 = 1.625. So yu will - theoretically - get 62.5% extra running.

The minimum "C" rating would depend on the motor or motor/prop  combination in the case of airplanes. Beyond a certain point there is no real benefit in going for a higher number. You will need to know the maximum energy consumption of your particular set up to get to this minimum figure. The manual would be a good source for this info.

mAh is the charge holding capacity of a battery, it's like  a fuel tank . The more/bigger you have the more run time you get. No increase in speed or acceleration here.

1s, 2s, 3s   Denotes number of cells in the li-ion battery, gives overall Voltage

25c, 35c.... C rating is the rate of discharging or the max amount of current your batteries can provide to your motor under load. So your motor will decide what ESC you'll have to use and that in turn determines the C rating that can support this setup.
As sushil has mentioned above, going beyond what's required doesn't yield any added advantage after a certain limit

Since you don't have to worry a lot about weight balance, measure your battery tray and go for the highest capacity lipo you can get.

Try and get 25 or 30C lipos. The ratings on lipos have never been verified and manufacturers tend to bloat values. Don't fall for the "150C" lipos. 30C should be good enough.

The fuel tank analogy is a good way to understand capacity.

hey there chintan, can you please tell me what batteries have you finally bought and from where ? thanks.