I've often read that opening up the exhause on the 2.5 can result in loss of lowend. So on my 02 2.5 wrangler I left the exhause alone. I needed all the lowend I could get. As for the 40 series, in my experience this muffler tends to resonate insde the vehicle at certain rpm,s. Might not be as much of an issue on a jeep, with the top off you hear everything any way. Between the wind and the muffler should about make your sterio useless without a kick butt system I would think. Let us know if you do loose some lowend or not, I'm curious about that. They say it helps the 6, but hurts the 4.
1987 wrangler, 258 I-6, 5-speed.
2007 liberty sport 4X4, automatic
2009 ford ranger 4x4 FX4 OFF/ROAD, 5-speed