I am of the opinion that Cessna would not have offered performance data for many of their older models, because their certification basis does not require that informaton to be furnished to the pilot. It is only later cert basis aircraft types which are required to provide this data. The one page performance page in the POH of the Cessna 150E, for which I am presently preparing a flight test plan, offers somewhat more than the minimum! A careful review of the cert basis may reveal that surprisingly little is required to be shown in respect of perfomance for a mod on a Cessna 172.
A rather favourable implicaton of this is that if your mod involves a performance penalty, it might slip through the approval process more easily, as you need only demonstrate compliance with the cert basis design requirements, rather than demonstrate that you have not affected performance.
I hope this helps,