the needle tends to be off about 300 rpm or so, when my car shifts at 6,000 it will go a little into the red as well, by the time the pcm tells the car to shift, the car really only will shift at 5800-5900 anyway.

belt slip becomes and issue when you get into the tiny pullies, i think you can run the stock size belt with a 3.4.

look at the boost gauge in this video

YouTube - Belt slip in action - 2.6" pulley