Note: If you increase the Wi-Fi output, but your device has the same output back to router, it doesn't really help now does it? I find this EXTREMELY useful though for using TWO DD-WRT Wireless Repeaters!
Go to Wireless >> WL# Advanced >> Advanced Settings >> TX Power
Insert new number, understand this WILL increase the heat your chip creates and therefor lower the lifespan of device. But if your like me, I get new hardware before anything breaks, and the Linksys EA6500 I use was under 50$ on eBay refurbished. I *think* that I have had errors @ 125 in summer where router just stopped working, and I unplugged it to cool down (It was HOT) and then plugged it back in and it worked again, that was before I had a second DD-WRT router to act as the AP for my repeater. Now I have them both at 100 and since it's not trying so hard to talk to the AP I can lower the repeaters TX down to 100 and keep temps down. Depends on where you live, what climate the routers will be in etc etc. AC Server Room, you should be OK. Hawaii in September, maybe not!