i vote use the new high flow cat, and i'll tell you why
1st. your car will sound like ass when you hit the big lobes with a straight pipe. at least put another resonator in its place.
2nd. at your hp levels, highly doubtful the ass dyno can feel the minimal power loss of a good cat over the straight pipe.
food for thought....my 12 yr old, 212k mile oem nissan cat, was flow benched to flow within ~15cu.ft./min. (or 5%) compared to a brand new 3"in/out highflow magnaflow universal cat.
3rd. earth day lasts all yr,..................................... for all of us.
do what YOU think IS RIGHT.
btw, nice "G"