It seems to me I've been reading about this subject forever... or however long I've been on these forums. From zero problems to bad enough to get rid of the car, the issue of an RPM-dependent engine-smoothness problem has either plagued or threatened a huge segment of the Subaru Community.
It has taken many years and an unorthodox approach, but a solution has been found recently. The links to the threads specifically dealing with this solution at the moment are:
Start here, with the post by XNWRX near the bottom:
As most know, the RR thread has been devoted to a software solution since 2009. While this newest revelation also involves changing and flashing a new map, it is a mechanical solution and the "tuning" is only necessary to remove the OEM ECU "crutch."
The change in my own, excellently running, engine performance was, in a word, astounding.