Just leave them stock. You will need to run with the car for awhile anyways to break in the new exhaust. I've found adjusting the J-pipes did very little, if anything. Not to mention they are a pain to adjust on the axlebacks. They are in a different place vs the catback and it was very difficult trying to get to them on the car. I had to remove the whole pipe as my breaker bar was too long and I couldn't get my socket wrench in there without hitting something. Much easier just taking off the whole J-pipe then adjusting (which required a pipe wrench to pull out the one side, couldn't move it by hand).
Save yourself the time, and just install them at factory position.
2008 G8 GT #509 of 888 - Ignition Orange
Performance: Kooks 1-7/8 LT's, Solo HFC & Mach Shorty, NEP OTR Intake, NEP Dyno Tune, VMS HSRK, Ported TB, Widened Stock 19" Wheels w/ 285's
Appearance: 5000K HIDs, NEP Gauge Bezel, Auto-dimming Mirror, third_shift studios 888 Shifter Plaque, GXP Front Fascia & Diffuser, 3.5" Exhaust Tips
349 RWHP / 366 RWTQ
Best 1/4 - 13.215 @ 105.17MPH, 60' - 2.01 - Prior to headers...new time coming (hopefully)