akry,
I'm not sure I'd believe 35HP without any other mods... 10-15HP is realistic, if coupled with a low restriction filter like K&N. Especially on the smaller 4.2 liter engine.... If you did a matched chip and airbox at the same time you might get 35HP.

If you are doing the airbox swap just make sure there isn't any dog-food between the airbox housing and the intake manifold. The first major service the 500E had there were about two measured cups of the stuff in there. In reading recent posts I see I'm not the only one...

Hope this helps...Lee