DURING DESCENT AT 215 MPH THE ACFT ENTERED VIOLENT BUFFETING. RECOVERY WAS INITIATED BY REDUCING POWER & THE ACFT LANDED WITHOUT ANY FURTHER INCIDENT. EXAMINATION REVEALED THE HORIZONTAL STABILATOR WAS PERMANENTLY BENT DOWNWARD ON BOTH SIDES. THE ACFT WAS MODIFIED WITH GAP SEALS ON 5/25/82 IAW STC SA516GL. EXAMINATION OF THE COUNTERBALANCE ARM ASSEMBLY SHOWED THAT TWO SMALL WEIGHTS (SLUGS) WERE USED TO OBTAIN A BALANCED CONDITION WHEN THE STC MODIFICATION WAS COMPLETED ON THE STABILATOR. THE STC CONTAINED A LIMITATION THAT IT SHOULD NOT BE INCORPORATED IN ANY ACFT HAVING OTHER APPROVED MODIFICATIONS UNLESS IT HAS BEEN DETERMINED THAT IT WILL NOT INTRODUCE ANY ADVERSE EFFECT UPON ACFT AIRWORTHINESS. THIS ACFT HAD 8 OTHER MODIFICATIONS. ON 7/10/82 THE OWNER OF THE STC ISSUED A SERVICE BULLETIN WHICH EFFECTIVELY REMOVED THE GAP SEALS ON ALL PA-30 & PA-39 TYPE.ACFT THAT HAD PREVIOUSLY BEEN MODIFIED.