Currently there is no aftermarket thermostat replacement other than the STETT kit. Your best bet is STETT OC stage 1.
Have you thought about the additional radiator from the performance package? Parts only cost $375 from www.getbmwparts.com
, then get a shop to install it for $100-200. You might think that this radiator won't do much, but it runs full-time no thermostat involved.. Might be worth a shot.