A downgrade will be extremely messy and time consuming, but you should not need to reinstall Debian...
Purge KDE and it's dependencies, then install a basic window manager (if you haven't already), startx and fire up synaptic (a program I would not normally recommend) and sort packages by origin. Then select all of those remaining packages from that repo and purge the lot.
$ aptitude search '~i ?origin(name_of_siduction_repo)'
I've no idea what the specific siduction repo is called, perhaps "kdenext"?
(The results of "apt-cache policy" will show this anyway.)
The command should show everything you have installed from that siduction repo and if you're confident you can purge the lot.
Once you're done:
$ apt-get clean && apt-get update
Then reinstall KDE from testing.
Personally I don't see any difference between any of the KDE's after 4.5.x'ish except the desktop wallpaper...... what problems are you having which makes you want to downgrade?