strtolist2 = Map[FromDigits, StringSplit[StringData[[All, 2]], ","], {-1}]

or make ``Internal`StringToDouble`` `Listable`:

 istd = Internal`StringToDouble; 
 SetAttributes[istd, Listable] ;

 strtolist3 = IntegerPart @ Map[istd, StringSplit[StringData[[All, 2]], ","], {-1}];

 strtolist3 == strtolist2 == strtolist
> True

Both are about twice as fast as the `For` loop with ``IntegerPart/@Internal`StringToDouble/@StringSplit[..]``