So this happened today.
Decided to get it on the Hub dyno for a comparison.
First run 600 with a misfire
two subsequent runs at 575 then this happened.
Early indications pointing to a dropped valve for some reason but I'll know more when I get the engine out and stripped in a week or two.
10,000 kms 16 drag runs and three dyno runs was due for a refresh but this wasn't quite what I had in mind.