I have avoided using packages primarily for this reason. One workaround that may work in some instances is to use a private folder and place the functions there. This limits who can call those functions to the parent folder only, but the functions themselves don't need any import or packagename. statements - they can call each other with abanadon. Now, if you want to expose those functions outside the package, you can create a wrapper as shown below.
Example:
C:\Temp\+TestPrivate\parentFun.m
C:\Temp\+TestPrivate\foo.m
C:\Temp\+TestPrivate\private\foo.m
C:\Temp\+TestPrivate\private\bar.m
Only functions within the package TestPrivate can call the private functions foo or bar, and foo and bar can call each other. Notice there's a package function called foo and also a private function called foo. The package function is simply a wrapper to the underlying private function (generic enough to be auto-generated). So, TestPrivate.foo() calls the private function foo(). It's content is as follows. As you can see, it should be fairly easy to auto-generate.
function varargout = foo(varargin)
% <Help copied from private foo>
[varargout{1:nargout}] = foo(varargin{:}); % This calls private\foo.m
Now how this affects performance is a difference question.
