Very interesting Paul, what manifold did you use? 4.5 is not a great improvement so I guess it was a 4-1 manifold,which is not best suited to the one/cooper,but often still fitted regardless.
Also the exhaust/manifold combo was not impressive,it does show that correct matching of components is very important,and simply buying and fitting a mixture of components all individually claiming" x" increase will not result in x + x + x ,they have to work in harmony with one another. I blame ebay
I think you will find the cylinder head restricts a lot of flow ,and trying to drag air through it with an 4-1 hard to say the least.
I do have a manifold here if you would like to try one and have the time to test it,
what diameter is the exhaust pipework? if its too big the one/cooper drops power -sounds good though .
I would expect 14 bhp off manifold and plus 6-8 with exhaust across the rev range
