Bad idea. It will be loud and sound like crap. Better off to get an offroad midpipe, you can get one on ebay for under $100, it'll sound better and benefit the car. As far as back pressure, losing the mufflers and keeping the cats won't really hurt it, but you won't like the result. Back pressure, while not an issue with your question, helps with scavanging and benefits lower rpm power.
The artist formerly known as SteedaMatt.