I bought the 3" Corsa cat-back for that very reason -- I often take trips in my GTS, and I didn't want to listen to the drone for 4-8 hours a day! I've had the Corsa installed now for 2 weeks, and I'm very pleased with the results. The car sounds great at idle, and great (loud) when I get on the throttle. But at cruising speeds in 5th and especially 6th gear -- no drone. In fact, clearly quieter than the Snake-Oyl rear pipe assembly I had before. My exact setup is 3" Corsa, Random Tech Hi-Flow cats, stock headers. And I have 3.55 gears, so my cruising RPM's are a little higher than a car with the stock 3.07. Incidentally, fit-and-finish on the Corsa was excellent.
My friend had a GTS with Belanger headers and exhaust, and no cats. It was LOUD! Too loud for me, anyway. Of course, exhaust sound is clearly a matter of personal preference.