I used 90 degree thermostat on my 400E this winter, my acutal coolant temperature was 100 degree, I then switched to OEM 80 degree thermostat since this February, my coolant temperature has been 90 degree since then.
Right after I switched thermostat, with 10 degree difference, I did notice slightly more power but my fuel economy got worse, from 14L/100km to 15L/100km, that was with winter gas though, in summer time I get much better than that.
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.