When i hit auto detect best settings ,it sets to low spec and my configs:i5 quadcore,4gb ram,
gt 550 1 gb is this really a low-end system or what is the use of configuration tool?

When i customise to mid- specs it runs between 6-36fps depending location,so
where is the enhancement?

all your drivers up to date?
i'm on an old school quadcore Q6600, 4 gigs ram, 9800GT and the config tool auto-detects to high-spec.. runs like ass with motion blur enabled, but 30+ fps without, at 1280x800

It autodetects like crap to be honest. Just try what settings that works out for you.
It detects my Geforce 560Ti as low, and HD6970 as low as well... and it detects the 7970 tested as medium. So take it with a grain of salt.

The enhancement was more content-based than it was performance-based. While they did patch a few performance issues, most of them affected either aesthetics or quests, not so much "general performance."

Your rig seems like it would run the game fine. Don't worry about scaling down, the game looks great on almost every setting. Not to mention, I ran on high-ultra spec for some time, not realizing that I was not getting the proper feedback during combat, and I was dying a lot due to control lag. I fiddled around with the settings and turned off the 'in-game' depth of field effect, and now it works great. I kept it on during cutscenes, as those seem to be generally less intensive.

For a full list of enhanced edition updates, check here:
http://www.evilavatar.com/forums/showthread.php?t=167963

This.