Its difficult to say what difference induction kits make in terms of overall power gains. What they will do is redistribute power through the revs. An ECU remap and a larger bore exhaust system together with an induction kit should give you a few more BHP at higher revs, but the low-end torque drops significantly. Means you have to work it harder when youre driving about town but foer B-road thrashing its a big improvement. In terms of the variations between induction kits, a K&N 57i will be good when theres a cold air supply (e.g. a ram pipe, filter relocation pipe so filter is in grille, lots of cold air feeds/dams, or cold weather) but when it gets warm it will lose power. A Piper-x is the same, but not quite as pronounced an effect. Viper kit is the least susceptible to temperature conditions as the carbon box practically shields the filter from all engine heat, but its a little more restrictive than an open IK so the potential gains are more limited. Also, it can result in knocking/pinking without a ECU remap to sort the fuelling.
Regarding your comment about the Powerflow exhaust not doing the car any justice. I assume what youre referring to is the fact that since its a custom made universal exhaust, rather than one designed for the specific application it wont be designed to the requirements of the individual car and therefore is likely to lose power, as the pipework may not be of the right bore/shape for the vehicle, therefore not attaining the best balance between back-pressure and free-flow for the individual vehicle. This is true up to a point, in that the main application specific exhaust companies design the exhaust for a car based on the properties of the OE system, and improve upon those with their replacement design.
But this does not mean that a Powerflow, etc. custom exhaust will not give power gains. If the combination of backbox and pipework is right theres no reason why not. Its just more of a lottery as to what you choose! An example of a car with a Powerflow running big power gains is Freds. As far as Im aware his IK is the same as Bambams, (S-Spec), and hes running 17"s (which will slow him down overall, the grip advantage is minimal), yet his times are not much different. And both cars are considerably up on standard power. So the Powerflow hes got does seem to be doing its job pretty well!