In russian old book “Pulsating stars” (Nauka, 1970, 372 pp.) the highest range of “dwarf cepheide” is noted as 1.7p for V879 Aql (page 264).
Surprisingly the star is EW according to VSX and GCVS, and there are not such vars in Aql.
It would be interesting to know, which SXPHE star actually has the largest pulsation amplitude.