I don't see why you would doubt what people have posted. Most of the information is from personal experience. And are not the manufacturer giving a claim to get a sale. The power gains are there.
Just in case you are not familiar with a dyno, it is a device used to measure horsepower/torque output. There are engine dynos and chasis dynos. My car was on a chasis dyno. So the hp numbers you will see are measured at the rear wheels (rwhp). I was told that to determine engine hp (bhp or fwhp) I divide the rwhp by .83. The dyno operator told me that the car has a 17% drivetrain loss.
Here is my car's response to headers then the flowmaster system. Which BTW is not the system for the most power. I chose it because of the deal I got on it, $300 installed(used). If I didn't care for the sound or the power gains it wasn't a total loss. I was sure I could sell it and break even. My car baseline dynoed at 328rwhp and 333rwtq.
I then added coated headers ($1100) and made another trip to the dyno. The result was 345 rwhp and 336 rwtq. I gained 17rwhp and 3rwtq. The cats were also removed at this time. Then came the flowmasters. The car dynoed at 357rwhp 344rwtq This next picture is a comparision headers (red) of the addition of the flowmasters(blue)
Now this is where it gets really interesting. Go back to the first picture. Look at how much power the car is making at 6700RPM. It is ~305rwhp. Now look at the blue line on the second picture, and at 6700RPM the car is making over 350rwhp. A gain of 45rwhp from the exhaust. Now translate that in fwhp, 45/.83 = 54 fwhp.
So for a $1400 modification I picked up ~30rwhp and 10rwtq at peak power output.
Sorry to make this so long winded, but you asked about a 50 hp increase in your original post. Not about should I get a c5 instead of modding my C4. If that was the question I would have just said get a C5. Doing the work and seeing the results is the second best part of doing the modding.

is the best part.