Stirling engine calculations

[Swifty]

Hi all, just wondering if someone could help me clarify my calculations. I have seen an online stiriling engine calculator here

http://www.bekkoame.ne.jp/~khirata/acad ... implee.htm

and have typed in 0.8Mpa for mean pressure (presume this is the pressure inside the displacement chamber)
10cm3 for volume of expansion space
gas temp. expansion 140 degrees
gas temp. compression 0 degrees
Helium as the working gas

It gives a maximum output of 18.4 Watts! I'm guessing you would be lucky to convert 20% of this?

It seems Stirling engines are not that efficient? (granted gas temp. difference is small)?