Capt wrote:
If the exhaust can suck a bit more air through, surly it need bigger main jets for it to work?
Yes and no.

Stock mains are overly large for full throttle/max revs application so in that regime, enabling a bit more airflow (even through a stock filter) goes some ways towards taking advantage of the extra fuel on call from the mains.

The potential downsides are elsewhere in the throttle/rev range where the mod may lean things out a little.  Not dangerously so but there are better ways of setting up the balance of intake/exhaust flow to work with any end less restrictive end can, not just the Yoshi.