I flew on a B767-200 direct from Luton to Orlando, both flights were over 9 hours and the planes were full
And with what cargo?
The ability to use the extra space not utilised for pax luggage is a key consideration, and I expect that your average charterer would make less use of such that SilverJet would have to in order to make their model work.
The weight saving from the seat removal will not be significant
Tosh - it is extremely significant to carry that fewer passengers, baggage and associated carry-on.