Yes PO B is great but I was thinking more about Natsu's equivalent to Leixia's 6B+K, 3B, 44K etc., those quicker moves that aren't guard crushers.
Thanks FleshMasher, this is helpful. It seemed relatively easy for me to research Leixia's best guard burst moves, what move to burst on (44K, 1B+K, WS K) but for Natsu it doesn't seem as straight forward. I know Leixia has an inherently superior guard burst game, but knowing the best move, while playing as Natsu, to use when my opponent is on flashing red to maximise the follow-up window would be ideal.
EDIT: From your link it looks like the kuzukiri bomb cancel is the best burster in terms of the follow-up window.