I found a helpful thread in which a chap measured his 100L boiler's stainless shroud temperature: on the outside surface of the shroud it was about 300C when being run on a low heat (from a 4-ring Australian style burner turned low), so presumably it would be much hotter on the inside surface with the gas turned up high. Though that was around the bottom of the shroud, adjacent to where the flames would be; it fell to just 95c at the top of the shroud, which was sized to give a 1/2" gap and sealed to the boiler at the top to make a warm air blanket (rather than to act as a chimney).
Presumably an outlet at the top would draw hot gasses up more effectively, which could give more rapid heating via the sides, but would also be less efficient due to letting hot gas escape; perhaps theres some optimum compromise, or else one could open a flu whilst bringing things to the boil and close it when just maintaining the boil.
Cheers
Kev
Gas boiler shroud/windshield