Can't supply you with the answers from 1st hand but it is alledged that fuel economy goes up and performance goes up by large amounts, from memory circa 25 - 30% for both, but as you say, the costs is high, something like £4500.
John Chatham has taken a modified car out for a run and is impressed, so much so, that he says that he is interested in its developement. AH parts advertise with a graph on the back of Revcounter ( our Healey magazine ) and at 4576 revs, power at the rear wheel goes up from just over 80 to just over 95.
You pays your money and you takes your chances
Bob