Is there experimental data showing the energy savings of good building airtightness?

Yes. The Building Research Establishment in the UK has published an experimental study showing energy savings achievable with improved airtightness [1,2].

[1] R. Coxon, “Research into the effect of improving airtightness in a typical UK dwelling,” The REHVA European HVAC Journal-Special issue on airtightness, vol. 50, no. 1, pp. 24-27, 2013.

[2] D. Butler and A. Perry, “Co-heating Tests on BRE Test Houses Before and After Remedial Air Sealing,” Building Research Establishment.