I wonder how much real, as opposed to theoretical, performance advantage is given by the alloy head? My iron-headed Flash seems quite brisk off the mark, and is certainly at least as fast in both acceleration and top speed terms as the Velo Venom it replaced*, as well as offering a more relaxed back-roads ride. But that's in real world conditions, not on a timing strip.
*Surprisingly, it also seems to handle better (at least up to 70 mph or so) - I used to get a distinct wriggle on white lines / road grooves on the Velo whilst the Flash is rock steady. Suspension, bearings etc on both in good condition. On looking at the frame, the BSA does have a much more rigid looking structure whereas the Velo frame is very obviously derived from the rigid.
No doubt howls of protest from Velo owners but again this is my experience of 2 specific bikes.