More likely to see oil fumes from the exhaust if your highflow cats (regardless of brand) do not have catalytic converters. There's some oil burnt even in the OEM setup, but you have 3 cats to deal with it, so you don't normally see much if anything. In addition, there's the debate about insufficient backpressure with catless downpipes causing turbo seals to blow out. That's why it's important to be sure which type you have on the car now.
Also check the intake piping and intercooler for pooling oil/crud. Not a given problem on the SHO, seems a lot of them stay "dry", but some don't.