Using a HEPA inner bag on a Kirby that was intended to use a paper bag will indeed cut back on the suction. The HEPA bags do not exhaust air as well as paper. So being the tradition has an already undersized fill tube, you will simply make it worse. Keeping the original fill tube is not possible.
You say you like the tradition fill tube because it is unique, and I respect that. However that does not mean that you need to discard it, you can simply install a heritage emptor, fill tube, and a heritage II bag connector, then install your tradition bag onto it. Save your old parts. If you don;t like the grey emptor, peel the paint off, wetsand the aluminum and buff out to a shine.
Vacuums are made to work, so therefor you sometimes have to modify them to make them better. Just like a 4 row, ball bearing brush roll is superior, yet there are collectors that refuse to run them because they are not original. Does it not make sense to wrap up the original 2 row and use the new brushroll to vacuum with?