Depends really you are right in what you are saying but you have to look into the figures. A car with plenum, ik and exhuast made 246.8 and with headers it made 247.9 so only 1bhp difference yeah? But it made that at peak. It gained 9.1bhp and some torque in the midrange. So its could be benifical if your going for lots of n/a power.
Most people are put off by the 4hours fitting time and some say its worth it and some say its too much £ per bhp. I suppose they are both right but depends on what your going for. A plus would be you exhuast would sound better.
Who told you 4 hours? I was quoted 10 hours for it & most people who have had them installed have said its atleast 8 hours
Decats are no more difficult to fit that a set of HFCs, except of course you have to remove them MOT time (unless you have a friendly MOT tester )
I had them on my car for a year & for the noise alone they are worth it, I'm currently looking into a removeable cat option