I think in previous discussions it was found that Isuzu did a pretty good job in mapping our ECU's and there was a not room for very much improvement (someone correct me if I am wrong). There were a few people who looked into this in the past.
As far as dual exhaust...IF done right it can give a HP boost. If done incorrectly it can cause HP decrease. The VX is not set up to well for a true cold air intake. You can open it up so that it breaths a little better though.
I am sure the power junkies will pitch in here shortly.