Chip, would a bell shape inside the filterneck be an improvement or would the difference be negligible? because it currently has a 90deg shoulder.
It would certainly extend the flow possible from the filter, but I doubt thats the bottle neck on standard cams etc so for that specific application I would be surprised if you saw any difference, might manage a bhp or two, but its so hard to actually accurately measure those sort of differences (rolling road runs on the same car vary that much between runs IME normally anyway)
If you have the space and you need a new filter, there is no downside really to fitting a bigger one, but like I said I wouldnt actually expect gains the one that came with the kit does seem to work very well despite not being massive.
Dan did 200bhp on one on his with 197 cams using that filter.