BTW, I've seen the VintageSpeed exhausts and the quality is excellent. My borther bought one but it will not fit on his Rallybug as it hangs a little too low ("sump guard" in the way). So it will go on his '64 instead
I haven't seen it in action so can't comment on the performance but the quality of the welds, tailpipes etc is very good.
Note, the vintagespeed exhausts are not merged exhausts but as per my previous comment I think a fully merged collector can be overrated depending on the engine spec.
My family car is a 300bhp N/A estate and due to being RHD it doesn't have the fancy exhaust manifold pipework of the LHD version (steering column gets in the way).
The LHD version has 305bhp.
So the right sized pipework with max airspeed (not too small and not too large) can perform well and not lose too much over a merged collector. Of course, once you get crazy with ported heads, camshafts etc.... I suspect the extra draw of the collector can make a real difference.