I honestly don't have any experience with PCM upgrades, but here's why I'm skeptical. Anyone who's actually chipped their H3, feel free to correct me.
Why people chip:
1. To move the powerband into a specific RPM range.
On engines with fixed valve timing, the timing doesn't change from low to high RPMs. The engineers had to decide not only where to optimize the powerband but also how to make the rest of the powerband acceptable while balancing power and fuel economy.
Aftermarket PCMs eliminate the power/economy trade-off and optimize timing in a specific RPM range (usually high-end for racing or low-end for towing and off-road). The trade-offs are poor performance in the off-range and engine stress from a combination of power in the optimized range and neglect in the rest of the band. Some companies offer an in-cab controller so you can pick your powerband for racing, towing, cruising or whatever.
2. To change the shift points (automatic trans)
This changes the RPM where each gear shifts up or down ... the same concept as the Power/Economy button on some cars. For maximum economy, you shift sooner to keep the average RPMs as low as possible. For maximum power, you stay in each gear as long as possible to stay in the powerband. Plus, when you shift, you're right below the power band and ready to accelerate through it.
3. To firm up the shifting (automatic trans)
This completes each shift faster. The thinking is that the faster you get into the next gear, the quicker you can get to the powerband in the new ratio. But it isn't as smooth between shifts and puts more stress on the drivetrain.
4. Correct the speedometer
If you change your tire size, you can usually correct it by chipping.
Why I wouldn't chip (even if I had an automatic):
1. The I-5 has VVT (variable valve timing)
VVT dynamically changes valve timing across the RPM band. Since GM didn't have to sacrifice power or durability at particular RPMs, they could optimize it from top to bottom. This partly explains the I-5's flat torque curve, providing almost as much torque at 2800 as it does at 5600 RPM. Basically, the RPM optimization is already done. There might be some wiggle room to trade economy for power, but you would really have to know what you're doing for minimal gains imo.
2. If you're driving hard, you're already flooring the pedal
... which keeps the transmission in gear through the power band for free.
3. Firmer shifts? Cool, you saved 0.5 seconds
4. Ok I admit it ... correcting for 35" tires would be cool.
5. Why risk voiding a 100,000 mile warranty?
6. Gas is expensive. It costs too much to lose a lot of fuel economy for marginal power gain.
7. Seat of the pants dyno? Come on ... The few people on this board who have tried chipping or programming the PCM said they "felt" a difference, but provided no convincing evidence. Give me dyno results, track times, and mpg measurements.
8. Time in the shop. I know a guy who chipped his truck. He won't shut up about the power gains, but it's always in the shop with engine and tranny problems. But then again it is a Dodge ...
So yeah, I'm skeptical, but I'd like to be wrong here. I want more power same as everyone else.