My 2cents,
When PROPERLY set up and tuned, There will be a performance incraese on a stock motor.
What was said before about the cam not being big enough, Not exactly true. Smaller chokes, and the fact that A stock donk wont rev much past seven grand, means that they should be able to do the job fine. ( lower top revs meaning no loss from the smaller chokes)
Plus, with the A14 having the best rod:stroke ratio, cylinder filling wont be an issue.

However, getting them all set up right is kind of an art, requiring the right equipment.
just going out and buying a pair of a 150bhp race motor, and chucking them on, will lead to all manner of woe.