I emailed them within the past fortnight to ask about this. To their credit, they were very honest and said it won't make any real difference to power output but might smooth out the throttle response a bit. As I've never notices any problem with the throttle response, I have decided it's not worth it. My bike is a 2012 F700GS, by the way.
(I also read on another site and a different bike model some information that throws a bit of doubt on their "before and after" dyno figures - someone who went to Hilltop and got the graphs, told the former owner who then then said he'd already had it done! I cannot vouch for the accuracy of this in any way.)