When you beat a Pro boss you will unlock "all upgrades" on a bike. This means that the bike can now be upgraded to the max amount of 28 in all attributes (speed, pump and jump). This is the same as bike 7.
This means you can race on bike 1 (or any Pro bike), but at the same speed, pump and jump attributes as bike 7 (when fully upgraded). This allows players more choices when customizing their bike and gear. Once all Pro bikes are unlocked, they all have the potential to be just as fast as bike 7!