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Looking at the OP's signature, I see he has some supporting hardware to compliment a potential turbo upgrade. Doesn't really detail his "custom" exhaust work, or to what extent that entails. Also, suffice to say, him running 10.5" wide rims at all four corners isn't helping.
He also goes onto compare the F150 (which as we all know, has different/hardened internals, as well as different, larger turbos) so to compare what an F150 is doing, to a SHO, isn't necessarily appropriate either.
Has the OP run his SHO at the track with the mods currently installed? If not, should do so.
This will give him at least a baseline to work from, and a reasonable target range of times to head towards.
I would say simply slapping on the ATP's, and then adjusting the tune file to take advantage of them, he would really be testing the limitations of the SHO's internals. At least to the degree of expecting any longevity out of it.
Also, without any supporting fuels mods, and me being generous.....
I would really be surprised if he were to eek out a 12.2 at best.