DRG A NGT ARRIVAL, THE PLT REQUESTED A SPECIAL VFR CLNC TO THE ARPT & WAS ADZD THAT THE WX WAS PARTIALLY OBSCURED, VIS WAS 1 MI WITH FOG & THE WIND WAS CALM. AS HE CONTD THE APCH, ALL FLTS WERE ADZD THE VIS HAD DROPPED TO 1/2 MI. THE PLT REQUESTED CLNC FOR AN ILS RWY 7 APCH. AFTER BEING CLRD FOR THE APCH, HE WAS ADZD THE RVR WAS '2000 TOUCHDOWN . . . MID- POINT 1000 & ROLLOUT 1600.' CONTACT WITH THE ACFT WAS LOST AS IT APCHD THE ARPT. LATER, THE ACFT WAS FND WHERE IT HAD COLLIDED WITH TREES & CRASHED APRX 1400 FT LEFT OF THE RWY & 500 FT PAST THE THRESHOLD. AN INV REVEALED THAT INITIAL IMPACT WAS IN A LVL ATTITUDE. AN EXAM OF THE ACFT REVEALED THE FLAPS WERE FULLY RETRACTED & THE THROTTLE WAS FULL OPEN. PROP DAMAGE INDCD THERE WAS ROTATION AT HIGH RPM DURING IMPACT. RECORDS SHOWED THE ACFT HAD AN INOP TURN COORDINATOR. NO OTHER PART FAILURE OR MALFUNCTION WAS EVIDENT. A CHECK OF THE WT & BALANCE INFO SHOWED THAT WHEN THE ACFT CRASHED, IT WAS APRX 55 LBS OVER THE MAX GROSS WT LIMITATION & THE CENTER-OF-GRAVITY (CG) WAS APRX .5 INCH BEHIND THE AFT LIMIT.