Here EU pricing for a Radeon Hd6990 : http://www.amazon.fr/Asus-Graphique-Radeon-HD6990-EAH6990/dp/B004RBWLZK/ref=sr_1_1?ie=UTF8&qid=1332690100&sr=8-1
I know it's amazon and that it's from France but if you shop around in the EU you can get some pretty good pricing (depends on where you live).
I hope this helps you for which GPU can fit into a rather tight SFF case. Do consider the noise and heat issues as well though. You should make sure to use good fans in that case. I don't care what uses a PC is used for. I prefer to help everyone as this helps me learn more about other approaches to BTC mining rigs (as I'll be researching different types of rigs and which approach fits best for people).
So what if OP wants to use workplace electricity as well as his own? That's his responsibility.If he gets fired so be it. I'd personally recommend that you (OP) reconsider your plan on using that mining rig at work (even for the free utilities there) because the noise of that mining rig can be rather loud and annoy other people in your workplace as well.
Having a powerful GPU will also mean looking for a beefy PSU.This will be harder to find for SFF cases that it would for a regular desktop case. So consider that as well.Finding smaller parts to fit will not be easy at all OP.
Ethics aside for utility use,it is ultimately your responsibility OP to make these decisions for yourself,even if they seem (or probably are) stupid/wrong.