the resonance is not bad with straight pipe. The only time you have any problem is when going too slow for the gear, so let's say you are trying to increase speed at 45 MPH in 6th. You will get resonance then, but not while running 70+ MPH. If you are running 2k RPM, you will be doing 90 with very little exhaust noise in the interior. Soooo.. to make a long story short, if you are in the correct gear, you won't get it.... AND the only time i get any anyway is when accelerating, not while cruising.
PS: please keep in mind the straight pipe i am talking about is from the resonator back, not from the cats back. This makes the car sound very good, but helps keep the interior noise down.