No answers, but I have a similar build (98' 4V, Kenne Belle, telorvek, sniper-tuned), so you have me curious enough to follow.
On the full length headers - i've been toying around forever shopping for a deal on a set from FFR, but have a hard time parting with that much money (for hookers) and don't have the equipment to do a scratch set myself.
FWIW - I've read that, compared to j-pipes, the 4:4s cost horsepower on NA 4Vs - presumably from less back pressure. No idea on S/C or otherwise modded.
Scott Miller : WebScott.Net
#6532: Mk3.1, 4.6L 32V DOHC (98 Mark VIII LSC), Kenne Bell 1.5L, Sniper-tuned Telorvek, TR-3650, IRS, Konis