Here is a link to the FSE Boost Valve page, http://www.fuelsystem.co.uk/Power%20Boost%20Valve.pdf
It lists the benefits as:
*Suitable for Standard and Tuned Vehicles
*Quicker Throttle Response
*Throttle Hesitation (in most cases eliminated)
*Normal Idle (with adjustment) Retained.
*Normal Economy on Light Throttle
*General Fitting Instructions included
*Weight 300 Grams Approx.
* Manufactured to Original Equipment Specs
Nowhere does it claim to give any more peak BHP and it does not, simples.
As I said before I have personal experience of one. Had a Golf GTi, 2nd one actually but with all the emission kit fitted the later car was a shadow of the first one. Had a Superchip fitted by a local dyno company, they claimed 8 to 12 BHP. They did a power run before and after and it gave an extra 4 BHP but the CO readings were off the scale everywhere and it would have failed any MOT, they took it out and refunded me. They then suggested one of these boost valves on a sale or return basis plus a K & N filter. What a difference, no hesitation, no change to emissions, great (possibly better than first car) throttle response but only 3 BHP extra which was down to the K & N.
Kept car for 6 years after having it fitted, well worth the money but as for giving another 25 bhp you were had.