I think you're a tad naive, if you don't think Celtic and Rangers, playing in the PL would not have enormous pull. I'd go as far as to say I don't think any teams games would be "miles ahead" of them.
Outside the UK&I? Not so sure; would be significant, but outside the Glasgow derby (no longer old firm to many) I don't think it would be anywhere near the realms of say Arsenal or Chelsea, never mind the other two.