Wingman,
Your productivity is based on actual block, not flight time. Going faster does not reduce your productivity payment.
For them to squeeze another flight into your roster you still get productivity if the total block time crosses the threshold.
For us its an increase of 6 to 12 kts of TAS, changing from .84/.85 to .86. So best case, on a 7 hour trip you save 10min. But only if you weren't slowed down somewhere. So if you are restricted by the 900hr/365 or 100hrs/ 28days, 18 sectors would be needed to squeeze a KWI in. Hmmmm, maybe an old 330 roster, but they were not timing out were they?
If people are stupid enough to work on a day off a high cost index will not stop them.