Hi Badboy,
>I'm assuming that a 20% increase is a theoretical maximum
No, it's actual data from the Jumo 213A-engined Focke-Wulf Fw 190D-9.
The higher figure is not surprising, though: Propeller thrust drops with speed while exhaust thrust doesn't, so the contribution of exhaust thrust would be greater for the Dora which certainly was faster than the 1941 Merlin-powered test aircraft. (I have just learned that Hawker made very extensive flight tests with a Merlin XX engine, maybe your test was one of those?)
Additionally, one would have to check whether the contribution is expressed in relation to shaft power or to total thrust. The latter is more accurate and the basis for my figure, but the former is more natural if you're an engine designer :-) Using shaft power instead of thrust power would make the exhaust thrust share appear a bit smaller, of course.
As you pointed out, different engine designs can turn out different thrust figures depending on their characteristics. Apparently, the Jumo 213 turned out more thrust than the otherwise similar DB603, for example.
I haven't checked this, but it's my impression that the use of MW50 might lead to an over-proportional increase of exhaust thrust because it increases the mass flow beyond the amount necessary for combustion. (Does that make sense? Hm, it might be just the opposite :-)
Regards,
Henning (HoHun)