Kapt -
They are 25 min transits. 20 mins is a tad optimistic. There is VERY FEW of them blocked in the schedule. It has always been that way. I reckon the precentage of 25 min transits would be around 10%.
You will find them alot in the morning between the first and second flight. Normally there is no pre-existing delays yet and the cabin is relatively clean. You only ever see 25 min transits at out ports where everything seems to happen quicker anyway.
Haven't seen any 25 min transits blocked for the 320s - only on the 717.
25 min transits are very achievable - even on the 320s. I just don't think it would be clever scheduling as there would be no room for delay. All 320 tranists are blocked at 30 mins. The new schedule has some at 35 mins so maybe there has been some problems getting them truned around.
I have been on flights where we have turned a 320 around in 25 mins but I think the PTS would be hard to follow. They allow 17 mins for the free-for-all that is boarding but it only ever takes 6-8 mins at the best of times.