I would think it has more to do with the transmission. These are not heavy duty transmissions they put in cars and crossovers like the Journey. The bigger SUVs and pickups have a heavier duty transmission (Chrysler 545RFE five-speed transmission) for the increased weight and load. Notice the Durango (6,200 lbs) and Grand Cherokee (5,000 lbs) both sport the 3.6 Penastar, yet they have a higher rated towing capacity due to their transmission. So the increased horsepower wouldn't affect the ability of the transmission to haul a heavier load. The transmission is the key component in towing capacity. Hauling anything over 2000 lbs in the Journey requires a transmission cooler. You really shouldn't haul anything near the rated capacity of 3500 lbs. It is interesting to note that the Durango and Grand Cherokee have Trailer Sway Control (TSC) which senses trailer sway and takes action by automatically applying the right amount of braking to maintain control. They also have a TOW / HAUL button for improved towing performance during trailer towing, heavy-load hauling or driving in mountainous areas. The TOW / HAUL mode inhibits fifth gear and allows upshifts to fourth during higher speed / lighter load conditions. These features are nice to have on vehicles specifically designed for towing. The Journey shouldn't be used for anything other than light duty hauling.
