Probably the biggest reason to root and install custom ROM is the fact that you can update your phone with updated software from a newer phone. In this case, you can in fact turn your Galaxy S5 into Galaxy S6 by using a custom ROM like Sixperience ROM.
The Sixperience ROM has been ported straight from a real Galaxy S6, giving you all the new software features including Galaxy S6 theme engine, resizable floating windows, S6 apps, and much more. Of course, you get a nice AROMA graphical installer also, allowing you to choose from three different kernels (you can overclock using Boeffla or KToonsez kernel) and even choose from two cameras (Note 4 or S5).
This ROM can be installed on most S6 models including SM-G900F/G900I/G900M, Canadian SM-G900W8, T-Mobile SM-G900T, Korean SM-G900S/K/T, and few more. (AT&T SM-G900A should be supported soon.)
There are couple things not working like fingerprint sensor (as S6 fingerprint software is not compatible with S5 fingerprint sensor) and world clock widget but other than that, the ROM itself runs rather good with great battery life. If you are looking to turn your Galaxy S5 into an S6, definitely try out this ROM this week(end) and do let me know what you think!