DDWRT is a 3rd party firmware that offers mild to moderate speed improvement, on some routers it makes it more stable, and on all routers it gives you way more options then the factory firmware does.
What we also like about dd-wrt is that no matter if we have a Linksys, or netgear, or tp-link, etc, if you flash any of them to DD-WRT, then no matter what brand you have, all dd-wrt flashed routers have the same available settings with the same menu's etc and you can even transfer over your custom configurations.
Right now every brand and even model line has different firmware layouts with different options and features. It would be like if instead of Microsoft windows, each product line of each computer company had its own custom operating system; so imagine there being 3 different OS software for this years line of Dell PCs, and 4 different ones for HP, etc, etc.
In regards to 10gbps switches/nic:
Pay no attention to that, I was just stating for correctness that cat6 is only better then cat5 if you have the 10gb enterprise hardware. 10gbps equipment is thousands of dollars.