1. Burgess isn't confirmed to be the first mateShiliew is practically featless, so saying he's stronger than the first mate is a huge speculation. His performance in dressrosa wasn't that bad if anything he lost to someone with the same rank as him pretty badly but still it isn't anyone lower.
So by feats even though Burguess didn't look so good at the end, it's still better than Shiliiew.
2. Shiryu actually does have better feats than Burgess since he was equal to Magellan pre-TS (which puts him above someone who got low diffed by Sabo).