B763 FLC OVERSHOOT ALT WHEN ISSUED MISSED APCH DUE TO ATL RWY CONFIGN CHANGE.

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A LOT OF TSTM ACTIVITY IN ATL AREA. WE WERE ON FINAL APCH ABOUT 4 MI OUT. TSTMS IN AREA CAUSED WIND SHIFT AT ARPT. WE WERE GOING AROUND FOR AN APCH TO RWY 27L. ATC ISSUED GAR INSTRUCTIONS. THE PUBLISHED MISSED APCH ALT IS 3500 FT. THAT IS WHAT WE HAD IN THE MCP. DURING THE MISSED; ATC ISSUED 4000 FT AND THAT WAS SET IN. THEN ATC CAME BACK AND SAID JUST GO TO 3500 FT. THAT WAS SET IN. WE WERE IN AUTO GAR. WE WERE VERY LIGHT THAT NIGHT (ONLY 8 PAX). WHEN WE SHIFTED ALT; THE AUTOTHROTTLES ENGAGED IN A HIGHER PWR SETTING. AUTOPLT AND AUTOTHROTTLES WERE IMMEDIATELY DISCONNECTED AND AN ATTEMPT TO LEVEL OFF AT NEWLY ASSIGNED ALT WAS ACCOMPLISHED. WE BALLOONED TO 3800 FT AND WENT BACK DOWN TO 3500 FT. I TRIED TO LEVEL OFF WITHOUT TOO MUCH PAX DISCOMFORT. A GREAT DEAL OF FORWARD PRESSURE WAS NEEDED. ATC MADE NO COMMENTS AND NO REASON FOR NONSTANDARD MISSED APCH ALT. THE B767 AUTO GAR LIMITS CLB TO NO MORE THAN 2000 FPM. BUT WITH ALL THE ALT CHANGES; A DISCONNECT WAS NECESSARY. OUR EXTREMELY LIGHT WT WAS ALSO A CONTRIBUTOR.