I looks like David Ainley is selling a modified throttle body for the SPi.
This poster in Australia has one installed.
http://www.theminiforum.co.uk/forums/topic/346733-spi-45mm-throttle-body/ (http://www.theminiforum.co.uk/forums/topic/346733-spi-45mm-throttle-body/)
It will always be beneficial t have data to back up claims of improved performance. Supposedly he tried the SC system for SPi and was not satisfied.
Here is a link to his website with the larger TB:
http://classicminisjapan.com/new-parts/45mm-big-throttle-body/ (http://classicminisjapan.com/new-parts/45mm-big-throttle-body/)