I have a ported and polished, gasket matched, intake manifold that was going to go on my last MINI. Needless to say it didn't get installed. The machine work was all done by Thumper. He did a beautiful job on it. This will fit a supercharged MINI. $200
I have heard air flow gains pushing 5-12%, but I would think that would vary depending on mods. I think you would not even want to consider one unless you have at least a head, camshaft, header and cat-back it would help kinda tie everything together and realize your best gains. An engine is kinda like a air pump........ more air in + more air out = more power.
Oh, I know all about more air, etc. I have a cam/header, no head yet and waiting to find the right cat back, also have 440 injectors just waiting to go in once I'm ready for a tune.