I was wondering if anybody had actually done proper measurements with the respective tips, to see whether they make a difference. (Controlling for the notion that an effective seal is achieved.)
None that I've seen...
I think tips definitely make a difference in sound quality, if I'm inferring what you mean, correctly? That is, assuming a proper seal is achieved across the spectrum of shapes and materials, will there be quantifiable differences in sound between tips? I tend to think about this in terms of full size speakers... An IEM essentially works with your ear to form a speaker box, which would make the tips roughly equivalent to the baffling and cones
I tend to agree with Chodi that foam, while providing the best isolation in my estimation, can definitely muffle the sound if improperly applied. I don't like foam tips that don't have a non-compressible structure at the end, for instance... Otherwise I feel the bore can become smushed shut in an ear, resulting in a muffled or muddy sound. Proper fit is also essential to prevent the bore from smushing shut... A foam tip can't be too large. I'm not referring to the seal here, but whether or not the end of the tip will impact against the wall of the ear, and be smushed shut.
Silicone tips are the simplest way to achieve terrific, clear and impactful sound while also achieving a good seal. They are also faster to insert and listen, as foam must be compressed, inserted, and then allowed to slowly expand in the ear. Again I agree with Chodi that Spinfits are an excellent option. I also highly recommend picking up the Westone STAR fit sampler if one decides to go silicone. However the best isolation with foam is still superior to the best isolation with silicone in my estimation...
