I dont think you need a custom rom . Ive tested tether and its fast . I'm dl-ing things at the same speed as my work connection. Look for clockworkmod tether.apk on the playstore. Though you can cook a rom. Go to xda and look for h3llzdr0id. He has a build box that Warp and I are using. We had to adjust it to build from source but it has a copy of " kitchen" , "kernel tools", and a few other tools to help you cook and theme.
Results from speed test on tether. http://imgur.com/IcoO76X.jpg
Second test http://imgur.com/2oQ33Jv.jpg