Bos,
You're bringing back old memories, do they still call it 'Doc's Rock' on the way in to ISC. 'Doc' was a Brymon mature Captain, it became known as his rock because as long as he got a sighting of that rock he could land in, literally, any weather conditions.
The passengers travelling SOU/CI are actually passengers who would otherwise fly from a London etc. airport, they simply board the train in London, get off at SOU and fly, if SOU closed they'd probably fly from London etc.
I appreciate that PZE station to St. Just is perhaps a similar distance as Newquay station to NQY but passengers don't often think logically, PZE and St. Just are two different locations, 'Nookie' is a single location.
You mentioned little difference in block times between the Do228 and the Twotter, what might be more relevant is to compare fuel burn and operating cost comparisons, particularly the new Do228NG. Whilst the beasts have different engines with a more aerodynamic appearance and, particularly, with retractable gear surely the Do228 must burn quite significantly less fuel!
There would be a significant difference in block times if such (ex Brymon) destinations as NQY/PLH/EXT/BRS were also served!
It will be interesting to see what may develop, my guess would be that the Choppers will lose appeal, they've succedded so long because only rotary wing could operate in/out of PZE, they're losing that route monopoly operating to/from fixed wing airports.
P.S. The jetfoils could be great fun to ride on, if there is sea traffic congestion they didn't slow down nor stop, they'd do a 'go around', one time outside Ramsgate harbour, lots of little boats in the way, we did about three 360's, at full speed, before entering the harbour!