"...keeping the 7 sp cassette."

Man, I missed this tidbit also! Must be the post Christmas meal stupor.

If the 7S rear has a spacer between the casette and the hub flange that'll make room for an 8S cassette the whole shifter vs. drivetrain becomes iffy. Look for some 7S Sora shifters as the 8S cassette gear spacing is close, but not exactly the same. You may luck out and there'll not be a problem or you may be able to shift fine at one end of the cassette and not the other.

Brad