The problems I've had with passing emissions went a little like this: I bought the kit and installed it minus an EGR and associated vacuum hoses. Everything was running great and it passed the first year. The next year I went to get tested, they said I needed to have an EGR. I connected the EGR and it ran like crap, but luckily (unluckily?) we have inspectors who know their way around an engine. The guy took a good look at the kit and said they sent me the wrong throttle body - the ported vacuum that controls the EGR signal wasn't there and instead it was a constant manifold vacuum port. He said it was likely the wrong model throttle body.
I called Howell, and they said he must be wrong. At this point I'd sunk so much money into everything on that Jeep I was fed up and just wanted to drive it. So, I set it up to pass the visual, with the inspector telling me it wouldn't pass the tail pipe as it was but he'd pass the visual. Went home, plugged up the hose running to the EGR, and took it through the tail pipe test. Passed with flying colors, and even a year later it's still beating the readings I get from my '06 Subaru. Government genius at work
I don't really do trails, but we've got some pretty steep roads to climb here on the front range, and I've never had any problems.