Is there any difference in a bow that is 54@30 drawing to 28" and a bow that is tillered 48@28

Depends on how the limbs were built and there max draw length. Not sure how much performance one would loose.

If the bows are identical my guess is performance would be the same.

It really depends on the build and design of the limb. However there should not be a lot of difference. A bow that is really built draw specific for 28 inches would probably outperform the longer draw bow when built for the longer draw;. The performance would most likely be in the 5 fps range.

God bless you , Steve

It is a Black Widow LB

Assuming all else is equal, which it almost never is, it would depend entirely on the draw force curve.  It's not necessarily the case that 54@30 is going to be 48@28.  It could be as much as 51 on some bows.