Power curve is not the only thing that dictates how high the plane can fly it is also the aerodynamic qualities, i.e. in this case wing loading.
With the same power Spit and 109 the Spit would fly higher due to bigger wing. More thrust would mean more speed, more alt and more maneuvering options, not just flying straight.
I have been under impression that even Spit 1 could fly at 40k. However, the climb rate plots on Spit Perf pages seem to suggest that the climb rate of Mk1 falls to zero at 34-35k.
http://www.spitfireperformance.com/spit1vrs109e.html(So to what number does the IX's climb rate point to?)
Wikipedia:
"On 5 February 1952, a Spitfire 19 of No. 81 Squadron RAF based in Hong Kong reached probably the highest altitude ever achieved by a Spitfire. The pilot, Flight Lieutenant Ted Powles, was on a routine flight to survey outside air temperature and report on other meteorological conditions at various altitudes in preparation for a proposed new air service through the area. He climbed to 50,000 feet (15,240 m) indicated altitude, with a true altitude of 51,550 feet (15,712 m). The cabin pressure fell below a safe level, and in trying to reduce altitude, he entered an uncontrollable dive which shook the aircraft violently. He eventually regained control somewhere below 3,000 feet (900 m) and landed safely with no discernible damage to his aircraft. Evaluation of the recorded flight data suggested that, in the dive, he achieved a speed of 690 mph (1,110 km/h, Mach 0.94), which would have been the highest speed ever reached by a propeller-driven aircraft."
So, going up there was a serious business and I presume "service ceiling" meaning the maximum safe altitude (and practical), not necessarily the maximum attainable altitude.
Also the power at alt is mandatory for safe flight at such heights so stripping the aircraft of any needless weight seems sensible also from point of view of maximum climb rate. That is because if somebody enters your airspace very very high the radar system may notice it rather late, meaning that you need all the speed and climb ability to catch him before he leaves your airspace, unless you are already patrolling quite high.
-C+
