There are litereally thousands upon thousands of Bretionian Knights and Errants. Agincourt was a tactical disaster by the french, Foot knights in a marsh going uphill. In addition Trebuchets are the last word in anti-castle weaponry. The Empire don't seem to have siege cannons.
You're theroycrafting too much using the abilities of the units in games. Reading though the fluff will show you it will be a lot closer than you give Brets credit for. I don't think we can make the assumptions that Empire will be tactical geniuses whilst the brets will simply ride towards the in single file getting gunned down. I believe that is pressed in an all out war the Bretoinians would win.