Surely it's not a case of "proper" or "Halfords" horsepower - it's what you want of the car.
If it's a road car then I don't see anything wrong in an exhaust to bring out the note and a superchip to make a bit of an improvement to driveability. Neither cost a great deal and neither add a huge amount to the car. Why? Well basically 255 from 3-litres means that the engine is already quite efficient from a naturally aspirated lump. Add 20bhp from Halfords and you are close to the magic 100bhp per litre that is the difficult one to breach cheaply.
Much over 275bhp N/A starts to cost disproportionately both in financial terms and driveability. Cost increases because trick materials are needed - steel cranks, forged pistons etc and driveability suffers when the power band get squeezed to accomodate wilder overlap on the cams.
On a road car, power figures don't matter anywhere near as much as torque and if I ran a V6 on the road and wanted serious power I'd go turbo/supercharger all day long with Nos as an addon if it still wasn't enough. I'd not spend as much as quoted here and I'd have a power curve that I could use. Trying to drive a car that works at high revs on the road and still get the best out of it is fraught with problems, not least the amount of commitment needed to drive on the limit. Forced induction doesn't put anywhere near the internal strain on the motor internals and works best from low down.
All this assumes that you can do something with the handling and brakes and transmission to contain it all - my guess is that ultimately you can't, and that much beyond the Halfords specials (if that's what we are now calling them!) there's little point spending on a V6 as you could spend the money more wisely on an M3, Scooby or Evo.
Unless that is, you think that the car is somehow "interesting" as an engineering challenge or a curio.
Which is why I no longer have a Japanese hairdyer