The performance will improve if you have the same memory module of the same speed but you wont see a drastic changes, you can have the single ram or two of 8gb ram. The brand does not necessarily matter most of the times but is required to have the same DDR.
I've found the specs of your laptop inspiron 15-5558
slots Two SODIMM slots
Type Dual-Channel DDR3L
Speed 1600 MHz
Configurations supported 2GB, 4GB, 6GB, 8GB, 12GB and 16GB
Here's the pdf specs for the laptop
http://www.europc.co.uk/media/specs/dell-inspiron-15.pdf