Well, it definitely boosted my signal. Tell me how to carry out a definitive test and I'll do it.
EDIT
I realise I won't have the necessary equipment to do anything scientific, so I downloaded a decibel meter app and chose an amp/cab combo, set all tone to zero and volume/gain to half way.
There was a 10 decibel increase according to my probably not very accurate app.
But really it could be louder it could be quieter.
Setting it up how I like it to sound and it increased by just shy of 20 dB.