I've towed 4,000 lbs to 5,000 lbs several times. Even been on Interstate 40 doing 75 mph with that much weight. The picture below is from several years ago when my 2006 was bone stock. Towed the Talon just fine doing 65 mph on the Hwy.
The 3.7 coil are essentially the same coil packs as used on the LS engines. They are just packaged different for the Atlas engines. There are people using them with the Vortec 4200 engine making over 800 HP with those coil packs.
Just a run down of some of the upgrades as of right now.
PCM of NC tune for 93 octane
Jags that run header (Sanderson Headers)
Custom CAT back exhaust
Air Doc intake
K&N air filter
Mishimoto oil catch can
Oil gressure guage
Brisk USA P5 spark plugs
Super sparks coil spring delete
There is people starting to mess with the 5 cylinders. They also have a bad rep for absolutely no reason. There is a guy in Michigan that will upgrade the 5 cylinder heads for like $1,300. He will port and polish it and put in bigger valves with heavier springs and re surface the head...
What year is your H3? The P12 PCM in the 2006 is completely differnt from the PCM in the 2007 and newer. I know in the 2008 to 2009 Trailerblazer the PCM went to speed density tuning. I doubt you will find anyone who knows. But your best bet is to send a guy named Calvin a message his...