Ace's strength

[ahill1]

Via scaling from his feats against Jimbei, it doesn't seem Ace is that much stronger than current G2 Luffy... but considering he was the 2nd commandant of Whitebeard's crew, 2nd just to Marco, wouldn't that imply he was a lot stronger, considering Luffy needed Gear Fourth to deal with the commander of a weaker Yonkou? I don't see why Whitebeard, the strongest Yonkou, would have his commanders weaker than any other Yonkou, either.

 

[Captain Cadaver]

Curiel confronted Moria without the latter getting oneshotted, so it's safe to say not all the WB are equal or even close.

I'd still say Ace ought to be fairly relevant, seeing as how he was able to stalemate Aokiji despite it being made apparent that DF abilities/counters become meaningless if your Haki is stronger. It's pretty easy to assume that Ace's growth was akin to Luffy's, meaning the version we see in the main series is most likely far stronger than the one who tied with Jimbei.

 

[sei'taer]

There's nothing to say the whitebeards commanders are ordered in strength or anything.

Burgess is blackbeards first captain, and I doubt he's currently the 2nd strongest.