Don't pay too much for them, but to answer your question they do help out a little but not much. You wont notice much, but I gained a couple HP on the dyno. My dyno tuner reccomended that with my mods I should get some after a dyno run and some current testing and what not. So I drove to a place a few miles away found some, came back and got my tuning done. I was happy with my results.
I think 8 guage will be fine. The biggest deal is to make sure your connections to the chassis, engine, etc are totally paint and rust free. You want nice shiny metal to shiny metal. You can put a little anti-oxidant compound on there for extra protection. The biggest fault with the factory wiring is that the chassis is painted then the ground wires are put in which leads to a marginal connection point. Making sure your connections are good will get you the best results. Wire gauge is less important. #8 or #6 will do just fine. This has been discussed to DEATH on some other forums I have frequented, and if I remember correctly, the dyno results from a stock wiring job that had the ground connections cleaned and tightened to a full aftermarket grounding kit were pretty minimal, like a couple of HP which may or may not have been just variances in the dyno itself.